diff --git a/atompack-py/python/atompack/__init__.pyi b/atompack-py/python/atompack/__init__.pyi
index a28c97e..9d49536 100644
--- a/atompack-py/python/atompack/__init__.pyi
+++ b/atompack-py/python/atompack/__init__.pyi
@@ -315,7 +315,7 @@ class Molecule:
         """
         ...
 
-    def get_property(self, key: str) -> float | int | str | npt.NDArray:
+    def get_property(self, key: str) -> float | int | str | npt.NDArray | None:
         """
         Get a custom property by key.
 
@@ -326,7 +326,7 @@ class Molecule:
 
         Returns
         -------
-        float, int, str, or ndarray
+        float, int, str, ndarray, or None
             Property value
 
         Raises
@@ -336,11 +336,11 @@ class Molecule:
         """
         ...
 
-    def set_property(self, key: str, value: float | int | str | npt.NDArray) -> None:
+    def set_property(self, key: str, value: float | int | str | npt.NDArray | None) -> None:
         """
         Set a custom property.
 
-        Supported types: float, int, str, 1D float32/float64/int32/int64 arrays,
+        Supported types: None, float, int, str, 1D float32/float64/int32/int64 arrays,
         and 2D float32/float64 arrays with shape (n, 3). Input dtype is preserved.
         The key 'stress' is reserved; use the dedicated ``stress`` property instead.
 
@@ -348,7 +348,7 @@ class Molecule:
         ----------
         key : str
             Property key
-        value : float, int, str, or ndarray
+        value : float, int, str, ndarray, or None
             Property value
 
         Raises
diff --git a/atompack-py/python/atompack/ase_bridge.py b/atompack-py/python/atompack/ase_bridge.py
index b268580..a1cc155 100644
--- a/atompack-py/python/atompack/ase_bridge.py
+++ b/atompack-py/python/atompack/ase_bridge.py
@@ -21,6 +21,7 @@
 _ASE_RESERVED_ARRAYS = {"numbers", "positions"}
 _ASE_TYPES = None
 _CALC_MODES = {"singlepoint", "nocopy", "none"}
+_UNSUPPORTED_PROPERTY = object()
 
 
 def _voigt6_to_mat3x3(stress):
@@ -53,6 +54,8 @@ def _get_stress(atoms):
 
 
 def _coerce_property(value, n_atoms):
+    if value is None:
+        return None
     if isinstance(value, (str, bool, int, float, np.integer, np.floating)):
         if isinstance(value, str):
             return value
@@ -76,7 +79,7 @@ def _coerce_property(value, n_atoms):
         if arr.dtype == np.float32:
             return arr.astype(np.float32, copy=False)
         return arr.astype(np.float64, copy=False)
-    return None
+    return _UNSUPPORTED_PROPERTY
 
 
 def _merge_properties(properties, builtins, values, n_atoms):
@@ -93,7 +96,7 @@ def _merge_properties(properties, builtins, values, n_atoms):
                     builtins["stress"] = arr.astype(np.float64, copy=False)
             continue
         coerced = _coerce_property(value, n_atoms)
-        if coerced is not None:
+        if coerced is not _UNSUPPORTED_PROPERTY:
             properties[key] = coerced
 
 
@@ -187,7 +190,7 @@ def _extract_ase_record(
             if key in _ASE_RESERVED_ARRAYS or key in _BUILTIN_FIELDS:
                 continue
             coerced = _coerce_property(value, n_atoms)
-            if coerced is not None:
+            if coerced is not _UNSUPPORTED_PROPERTY:
                 properties[key] = coerced
 
     calc = getattr(atoms, "calc", None)
@@ -196,7 +199,7 @@ def _extract_ase_record(
         for key, value in results.items():
             if key not in _BUILTIN_FIELDS:
                 coerced = _coerce_property(value, n_atoms)
-                if coerced is not None:
+                if coerced is not _UNSUPPORTED_PROPERTY:
                     properties[key] = coerced
 
     if copy_info and getattr(atoms, "info", None):
diff --git a/atompack-py/src/database_flat.rs b/atompack-py/src/database_flat.rs
index a8ada30..8486633 100644
--- a/atompack-py/src/database_flat.rs
+++ b/atompack-py/src/database_flat.rs
@@ -368,6 +368,9 @@ pub(super) fn get_molecules_flat_soa_impl<'py>(
                         }
 
                         if schema_entry.slot_bytes == 0 {
+                            if schema_entry.type_tag == TYPE_NONE {
+                                continue;
+                            }
                             if let Some(ref mtx) = string_mutexes[section_idx] {
                                 let val = Some(
                                     std::str::from_utf8(sec.payload)
@@ -449,6 +452,9 @@ pub(super) fn get_molecules_flat_soa_impl<'py>(
                         }
 
                         if schema_entry.slot_bytes == 0 {
+                            if schema_entry.type_tag == TYPE_NONE {
+                                continue;
+                            }
                             if let Some(ref mtx) = string_mutexes[section_idx] {
                                 let val = Some(
                                     std::str::from_utf8(sec.payload)
diff --git a/atompack-py/src/lib.rs b/atompack-py/src/lib.rs
index 3e6b6e3..92639ea 100644
--- a/atompack-py/src/lib.rs
+++ b/atompack-py/src/lib.rs
@@ -113,6 +113,7 @@ const TYPE_BOOL3: u8 = 9;
 const TYPE_MAT3X3_F64: u8 = 10;
 const TYPE_FLOAT32: u8 = 11;
 const TYPE_MAT3X3_F32: u8 = 12;
+const TYPE_NONE: u8 = 13;
 
 const RECORD_FORMAT_SOA_V2: u32 = 2;
 const RECORD_FORMAT_SOA_V3: u32 = 3;
diff --git a/atompack-py/src/molecule_helpers.rs b/atompack-py/src/molecule_helpers.rs
index b48c3e8..12aa81a 100644
--- a/atompack-py/src/molecule_helpers.rs
+++ b/atompack-py/src/molecule_helpers.rs
@@ -374,6 +374,7 @@ pub(super) fn property_value_to_pyobject(
     value: &PropertyValue,
 ) -> PyResult<Py<PyAny>> {
     Ok(match value {
+        PropertyValue::None => py.None(),
         PropertyValue::Float(v) => into_py_any(py, *v)?,
         PropertyValue::Int(v) => into_py_any(py, *v)?,
         PropertyValue::String(v) => into_py_any(py, v)?,
@@ -411,6 +412,12 @@ pub(super) fn property_section_to_pyobject<'py>(
         Ok(payload.len() / stride)
     };
     Ok(match section.type_tag {
+        TYPE_NONE => {
+            if !payload.is_empty() {
+                return Err(PyValueError::new_err("Null property payload must be empty"));
+            }
+            py.None()
+        }
         TYPE_FLOAT => into_py_any(py, read_f64_scalar(payload)?)?,
         TYPE_INT => into_py_any(py, read_i64_scalar(payload)?)?,
         TYPE_STRING => into_py_any(
@@ -448,6 +455,7 @@ pub(super) fn property_section_to_pyobject<'py>(
 
 fn property_value_is_atom_array(value: &PropertyValue, n_atoms: usize) -> bool {
     match value {
+        PropertyValue::None => false,
         PropertyValue::FloatArray(values) => values.len() == n_atoms,
         PropertyValue::Vec3Array(values) => values.len() == n_atoms,
         PropertyValue::IntArray(values) => values.len() == n_atoms,
diff --git a/atompack-py/src/py_dtypes.rs b/atompack-py/src/py_dtypes.rs
index e05f112..95a0bd3 100644
--- a/atompack-py/src/py_dtypes.rs
+++ b/atompack-py/src/py_dtypes.rs
@@ -243,6 +243,9 @@ pub(crate) fn parse_mat3_field(value: &Bound<'_, PyAny>, label: &str) -> PyResul
 }
 
 pub(crate) fn parse_property_value(value: &Bound<'_, PyAny>) -> PyResult<PropertyValue> {
+    if value.is_none() {
+        return Ok(PropertyValue::None);
+    }
     if let Ok(v) = value.extract::<i64>() {
         return Ok(PropertyValue::Int(v));
     }
@@ -305,6 +308,6 @@ pub(crate) fn parse_property_value(value: &Bound<'_, PyAny>) -> PyResult<Propert
         });
     }
     Err(PyValueError::new_err(
-        "Unsupported property type. Supported: float, int, str, ndarray",
+        "Unsupported property type. Supported: None, float, int, str, ndarray",
     ))
 }
diff --git a/atompack-py/src/soa.rs b/atompack-py/src/soa.rs
index 793df97..d496c9b 100644
--- a/atompack-py/src/soa.rs
+++ b/atompack-py/src/soa.rs
@@ -150,6 +150,7 @@ pub(crate) fn section_schema_from_ref(
 ) -> atompack::Result<SectionSchema> {
     let per_atom = is_per_atom(section.kind, section.key, section.type_tag);
     let elem_bytes = match section.type_tag {
+        TYPE_NONE => 0,
         TYPE_STRING => 0,
         tag if per_atom => {
             let elem_bytes = type_tag_elem_bytes(tag);
@@ -168,7 +169,7 @@ pub(crate) fn section_schema_from_ref(
         TYPE_MAT3X3_F64 => 72,
         _ => section.payload.len(),
     };
-    let slot_bytes = if section.type_tag == TYPE_STRING {
+    let slot_bytes = if matches!(section.type_tag, TYPE_STRING | TYPE_NONE) {
         0
     } else if per_atom {
         elem_bytes
@@ -196,6 +197,15 @@ pub(crate) fn validate_section_payload(
     n_atoms: usize,
 ) -> atompack::Result<()> {
     match section.type_tag {
+        TYPE_NONE => {
+            if !section.payload.is_empty() {
+                return Err(invalid_data(format!(
+                    "Section '{}' has invalid payload length {} (expected 0)",
+                    section.key,
+                    section.payload.len()
+                )));
+            }
+        }
         TYPE_STRING => {
             std::str::from_utf8(section.payload)
                 .map_err(|_| invalid_data(format!("Invalid UTF-8 in section '{}'", section.key)))?;
@@ -254,6 +264,7 @@ pub(crate) fn validate_section_payload(
 /// Element size in bytes for a given type tag. Returns 0 for variable-length types.
 pub(crate) fn type_tag_elem_bytes(tag: u8) -> usize {
     match tag {
+        TYPE_NONE => 0,
         TYPE_FLOAT => 8,
         TYPE_INT => 8,
         TYPE_STRING => 0,
@@ -289,7 +300,7 @@ fn database_schema_section(
     n_atoms: usize,
 ) -> PyResult<DatabaseSchemaSection> {
     let per_atom = is_per_atom(kind, key, type_tag);
-    let elem_bytes = if type_tag == TYPE_STRING {
+    let elem_bytes = if matches!(type_tag, TYPE_STRING | TYPE_NONE) {
         0
     } else {
         let elem_bytes = type_tag_elem_bytes(type_tag);
@@ -301,7 +312,7 @@ fn database_schema_section(
         }
         elem_bytes
     };
-    let slot_bytes = if type_tag == TYPE_STRING {
+    let slot_bytes = if matches!(type_tag, TYPE_STRING | TYPE_NONE) {
         0
     } else if per_atom {
         let expected = n_atoms.checked_mul(elem_bytes).ok_or_else(|| {
@@ -1096,6 +1107,12 @@ fn decode_mat3x3_f32(payload: &[u8]) -> PyResult<[[f32; 3]; 3]> {
 
 fn decode_property_value(type_tag: u8, payload: &[u8]) -> PyResult<PropertyValue> {
     Ok(match type_tag {
+        TYPE_NONE => {
+            if !payload.is_empty() {
+                return Err(PyValueError::new_err("Null property payload must be empty"));
+            }
+            PropertyValue::None
+        }
         TYPE_FLOAT => PropertyValue::Float(read_f64_scalar(payload)?),
         TYPE_INT => PropertyValue::Int(read_i64_scalar(payload)?),
         TYPE_STRING => PropertyValue::String(
diff --git a/atompack-py/tests/test_atom_molecule.py b/atompack-py/tests/test_atom_molecule.py
index 5674ca0..6c03f83 100644
--- a/atompack-py/tests/test_atom_molecule.py
+++ b/atompack-py/tests/test_atom_molecule.py
@@ -169,6 +169,7 @@ def test_molecule_custom_properties() -> None:
     mol.set_property("int_vec32", np.array([3, 4], dtype=np.int32))
     mol.set_property("vec3", np.array([[0.1, 0.2, 0.3], [0.4, 0.5, 0.6]], dtype=np.float32))
     mol.set_property("vec3_f64", np.array([[1.1, 1.2, 1.3], [2.1, 2.2, 2.3]], dtype=np.float64))
+    mol.set_property("optional_label", None)
     mol.stress = np.eye(3, dtype=np.float64) * 3.0
 
     assert mol.get_property("temperature") == pytest.approx(300.0)
@@ -208,10 +209,12 @@ def test_molecule_custom_properties() -> None:
     np.testing.assert_allclose(
         vec3_f64, np.array([[1.1, 1.2, 1.3], [2.1, 2.2, 2.3]], dtype=np.float64)
     )
+    assert mol.get_property("optional_label") is None
 
     np.testing.assert_allclose(mol.stress, np.eye(3, dtype=np.float64) * 3.0)
 
     assert mol.has_property("method") is True
+    assert mol.has_property("optional_label") is True
     assert mol.has_property("stress") is False
     assert set(mol.property_keys()) >= {
         "temperature",
@@ -223,6 +226,7 @@ def test_molecule_custom_properties() -> None:
         "int_vec32",
         "vec3",
         "vec3_f64",
+        "optional_label",
     }
 
     with pytest.raises(KeyError, match=r"not found"):
diff --git a/atompack-py/tests/test_database.py b/atompack-py/tests/test_database.py
index 4d83dfd..ccaac1a 100644
--- a/atompack-py/tests/test_database.py
+++ b/atompack-py/tests/test_database.py
@@ -20,6 +20,7 @@ def _make_molecule(energy: float) -> atompack.Molecule:
     mol.velocities = np.array([[1.0, 0.0, 0.0], [0.0, 1.0, 0.0]], dtype=np.float32)
     mol.cell = np.eye(3, dtype=np.float64) * 2.0
     mol.set_property("tag", "train")
+    mol.set_property("optional", None)
     mol.set_property("ids", np.array([1, 2], dtype=np.int64))
     return mol
 
@@ -48,6 +49,7 @@ def test_database_roundtrip(tmp_path: Path, compression: str) -> None:
     np.testing.assert_allclose(mol1_r.velocities, mol1.velocities)
     np.testing.assert_allclose(mol1_r.cell, mol1.cell)
     assert mol1_r.get_property("tag") == "train"
+    assert mol1_r.get_property("optional") is None
     np.testing.assert_array_equal(mol1_r.get_property("ids"), np.array([1, 2], dtype=np.int64))
 
     batch = db2.get_molecules([0, 1])
diff --git a/atompack-py/tests/test_from_ase.py b/atompack-py/tests/test_from_ase.py
index 8505310..c485cd4 100644
--- a/atompack-py/tests/test_from_ase.py
+++ b/atompack-py/tests/test_from_ase.py
@@ -87,6 +87,7 @@ def test_from_ase_extracts_core_fields() -> None:
             "int_vec32": np.array([3, 4], dtype=np.int32),
             "vec3": np.array([[0.1, 0.2, 0.3], [0.4, 0.5, 0.6]], dtype=np.float32),
             "vec3_f64": np.array([[1.0, 1.1, 1.2], [2.0, 2.1, 2.2]], dtype=np.float64),
+            "nullable": None,
             "stress": np.array(
                 [[1.1, 1.2, 1.3], [2.1, 2.2, 2.3], [3.1, 3.2, 3.3]], dtype=np.float64
             ),
@@ -126,6 +127,7 @@ def test_from_ase_extracts_core_fields() -> None:
     np.testing.assert_allclose(
         mol.get_property("vec3_f64"), np.array([[1.0, 1.1, 1.2], [2.0, 2.1, 2.2]], dtype=np.float64)
     )
+    assert mol.get_property("nullable") is None
     np.testing.assert_allclose(
         mol.stress,
         np.array([[1.1, 1.2, 1.3], [2.1, 2.2, 2.3], [3.1, 3.2, 3.3]], dtype=np.float64),
@@ -360,6 +362,22 @@ def test_to_ase_owned_maps_builtins_and_properties() -> None:
     np.testing.assert_array_equal(atoms.arrays["tags"], np.array([3, 4], dtype=np.int32))
 
 
+def test_to_ase_roundtrip_preserves_none_custom_property() -> None:
+    mol = atompack.Molecule.from_arrays(
+        np.array([[0.0, 0.0, 0.0]], dtype=np.float32),
+        np.array([1], dtype=np.uint8),
+    )
+    mol.set_property("nullable", None)
+
+    atoms = mol.to_ase()
+    assert "nullable" in atoms.info
+    assert atoms.info["nullable"] is None
+
+    roundtrip = atompack.from_ase(atoms)
+    assert roundtrip.has_property("nullable") is True
+    assert roundtrip.get_property("nullable") is None
+
+
 def test_to_ase_calc_modes() -> None:
     mol = atompack.Molecule.from_arrays(
         np.array([[0.0, 0.0, 0.0], [1.0, 0.5, 0.0]], dtype=np.float32),
diff --git a/atompack/src/storage/dtypes.rs b/atompack/src/storage/dtypes.rs
index 950b9b0..309eec1 100644
--- a/atompack/src/storage/dtypes.rs
+++ b/atompack/src/storage/dtypes.rs
@@ -69,6 +69,7 @@ pub(super) fn float_scalar_payload_len(value: &FloatScalarData) -> usize {
 
 pub(super) fn property_value_type_tag(value: &PropertyValue) -> u8 {
     match value {
+        PropertyValue::None => TYPE_NONE,
         PropertyValue::Float(_) => TYPE_FLOAT,
         PropertyValue::Int(_) => TYPE_INT,
         PropertyValue::String(_) => TYPE_STRING,
@@ -83,6 +84,7 @@ pub(super) fn property_value_type_tag(value: &PropertyValue) -> u8 {
 
 pub(super) fn property_value_payload_len(value: &PropertyValue) -> usize {
     match value {
+        PropertyValue::None => 0,
         PropertyValue::Float(_) | PropertyValue::Int(_) => 8,
         PropertyValue::String(value) => value.len(),
         PropertyValue::FloatArray(values) => values.len() * 8,
@@ -120,6 +122,7 @@ fn extend_i32(buf: &mut Vec<u8>, values: &[i32]) {
 
 pub(super) fn property_value_to_bytes(value: &PropertyValue) -> Vec<u8> {
     match value {
+        PropertyValue::None => Vec::new(),
         PropertyValue::Float(value) => value.to_le_bytes().to_vec(),
         PropertyValue::Int(value) => value.to_le_bytes().to_vec(),
         PropertyValue::String(value) => value.as_bytes().to_vec(),
@@ -291,6 +294,14 @@ pub(super) fn decode_mat3x3_f64(payload: &[u8]) -> Result<[[f64; 3]; 3]> {
 
 pub(super) fn decode_property_value(type_tag: u8, payload: &[u8]) -> Result<PropertyValue> {
     Ok(match type_tag {
+        TYPE_NONE => {
+            if !payload.is_empty() {
+                return Err(Error::InvalidData(
+                    "null property payload must be empty".into(),
+                ));
+            }
+            PropertyValue::None
+        }
         TYPE_FLOAT => {
             if payload.len() < 8 {
                 return Err(Error::InvalidData("f64 property truncated".into()));
diff --git a/atompack/src/storage/mod.rs b/atompack/src/storage/mod.rs
index 886aceb..405bb4c 100644
--- a/atompack/src/storage/mod.rs
+++ b/atompack/src/storage/mod.rs
@@ -79,6 +79,7 @@ const TYPE_BOOL3: u8 = 9; // [bool; 3]
 const TYPE_MAT3X3_F64: u8 = 10; // [[f64; 3]; 3]
 const TYPE_FLOAT32: u8 = 11; // f32 scalar
 const TYPE_MAT3X3_F32: u8 = 12; // [[f32; 3]; 3]
+const TYPE_NONE: u8 = 13; // explicit null property
 
 // Two redundant page-aligned header slots for crash safety.
 const HEADER_SLOT_SIZE: usize = 4096;
@@ -1425,6 +1426,8 @@ mod tests {
             "i32arr".to_string(),
             PropertyValue::Int32Array(vec![100, -200]),
         );
+        mol.properties
+            .insert("none_val".to_string(), PropertyValue::None);
 
         {
             let mut db = AtomDatabase::create(&path, CompressionType::Lz4).unwrap();
@@ -1463,7 +1466,7 @@ mod tests {
         }
 
         // Verify properties
-        assert_eq!(r.properties.len(), 9);
+        assert_eq!(r.properties.len(), 10);
         match r.properties.get("scalar_f").unwrap() {
             PropertyValue::Float(v) => assert_eq!(*v, 99.9),
             other => panic!("expected Float, got {:?}", other),
@@ -1476,6 +1479,10 @@ mod tests {
             PropertyValue::String(v) => assert_eq!(v, "hello"),
             other => panic!("expected String, got {:?}", other),
         }
+        match r.properties.get("none_val").unwrap() {
+            PropertyValue::None => {}
+            other => panic!("expected None, got {:?}", other),
+        }
     }
 
     #[test]
diff --git a/atompack/src/storage/schema.rs b/atompack/src/storage/schema.rs
index 0f87334..7ba950f 100644
--- a/atompack/src/storage/schema.rs
+++ b/atompack/src/storage/schema.rs
@@ -125,6 +125,7 @@ pub(super) fn decode_schema_lock(bytes: &[u8]) -> Result<SchemaLock> {
 
 fn schema_type_tag_elem_bytes(tag: u8) -> Result<usize> {
     match tag {
+        TYPE_NONE => Ok(0),
         TYPE_FLOAT => Ok(8),
         TYPE_INT => Ok(8),
         TYPE_STRING => Ok(0),
@@ -163,7 +164,7 @@ fn schema_entry(
 ) -> Result<SchemaEntry> {
     let per_atom = schema_is_per_atom(kind, key);
     let elem_bytes = schema_type_tag_elem_bytes(type_tag)?;
-    let slot_bytes = if type_tag == TYPE_STRING {
+    let slot_bytes = if matches!(type_tag, TYPE_STRING | TYPE_NONE) {
         0
     } else if per_atom {
         match type_tag {
diff --git a/atompack/src/types.rs b/atompack/src/types.rs
index 506e4af..e6d8b54 100644
--- a/atompack/src/types.rs
+++ b/atompack/src/types.rs
@@ -3,6 +3,7 @@ use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum PropertyValue {
+    None,
     Float(f64),
     Int(i64),
     String(String),