diff --git a/glotaran/builtin/models/kinetic_image/k_matrix.py b/glotaran/builtin/models/kinetic_image/k_matrix.py
index 3a47be566..c129f64d2 100644
--- a/glotaran/builtin/models/kinetic_image/k_matrix.py
+++ b/glotaran/builtin/models/kinetic_image/k_matrix.py
@@ -63,9 +63,9 @@ def combine(self, k_matrix: KMatrix) -> KMatrix:
         """
         if not isinstance(k_matrix, KMatrix):
             raise TypeError("K-Matrices can only be combined with other K-Matrices.")
-        combined_matrix = {entry: k_matrix.matrix[entry] for entry in k_matrix.matrix}
-        for entry in self.matrix:
-            combined_matrix[entry] = self.matrix[entry]
+        combined_matrix = {entry: self.matrix[entry] for entry in self.matrix}
+        for entry in k_matrix.matrix:
+            combined_matrix[entry] = k_matrix.matrix[entry]
         combined = KMatrix()
         combined.label = f"{self.label}+{k_matrix.label}"
         combined.matrix = combined_matrix
diff --git a/glotaran/builtin/models/kinetic_image/test/test_k_matrix.py b/glotaran/builtin/models/kinetic_image/test/test_k_matrix.py
index 128765e97..4f9065d83 100644
--- a/glotaran/builtin/models/kinetic_image/test/test_k_matrix.py
+++ b/glotaran/builtin/models/kinetic_image/test/test_k_matrix.py
@@ -274,3 +274,29 @@ def test_unibranched():
 
     print(mat.a_matrix_unibranch(con))
     assert np.allclose(mat.a_matrix_unibranch(con), wanted_a_matrix)
+
+
+def test_combine_matrices():
+
+    matrix1 = {
+        ("s1", "s1"): "1",
+        ("s2", "s2"): "2",
+    }
+    mat1 = KMatrix()
+    mat1.label = "A"
+    mat1.matrix = matrix1
+
+    matrix2 = {
+        ("s2", "s2"): "3",
+        ("s3", "s3"): "4",
+    }
+    mat2 = KMatrix()
+    mat2.label = "B"
+    mat2.matrix = matrix2
+
+    combined = mat1.combine(mat2)
+
+    assert combined.label == f"{mat1.label}+{mat2.label}"
+    assert combined.matrix[("s1", "s1")] == mat1.matrix[("s1", "s1")]
+    assert combined.matrix[("s2", "s2")] == mat2.matrix[("s2", "s2")]
+    assert combined.matrix[("s3", "s3")] == mat2.matrix[("s3", "s3")]