diff --git a/src/mcs/mcs_types.jl b/src/mcs/mcs_types.jl
index 059cf8c00..27e1b792b 100644
--- a/src/mcs/mcs_types.jl
+++ b/src/mcs/mcs_types.jl
@@ -161,6 +161,7 @@ mutable struct SimulationInstance{T}
     sim_def::SimulationDef{T} where T <: AbstractSimulationData
     models::Vector{Model}
     results::Vector{Dict{Tuple, DataFrame}}
+    payload::Any
 
     function SimulationInstance{T}(sim_def::SimulationDef{T}) where T <: AbstractSimulationData
         self = new()
@@ -168,7 +169,8 @@ mutable struct SimulationInstance{T}
         self.current_trial = 0
         self.current_data = nothing
         self.sim_def = deepcopy(sim_def)
-        
+        self.payload = deepcopy(self.sim_def.payload)
+
         # These are parallel arrays; each model has a corresponding results dict
         self.models = Vector{Model}(undef, 0)
         self.results = [Dict{Tuple, DataFrame}()]
@@ -189,7 +191,8 @@ end
 """
     set_payload!(sim_def::SimulationDef, payload) 
 
-Attach a user's `payload` to the `SimulationDef` instance so it can be
+Attach a user's `payload` to the `SimulationDef`. A copy of the payload object
+will be stored in the `SimulationInstance` at run time so it can be
 accessed in scenario and pre-/post-trial callback functions. The value
 is not used by Mimi in any way; it can be anything useful to the user.
 """
@@ -202,6 +205,13 @@ Return the `payload` value set by the user via `set_payload!()`.
 """
 payload(sim_def::SimulationDef) = sim_def.payload
 
+"""
+    payload(sim_inst::SimulationInstance)
+
+Return the copy of the `payload` value stored in the `SimulationInstance` set by the user via `set_payload!()`.
+"""
+payload(sim_inst::SimulationInstance) = sim_inst.payload
+
 struct MCSData <: AbstractSimulationData end
 
 const MonteCarloSimulationDef = SimulationDef{MCSData}
diff --git a/test/mcs/runtests.jl b/test/mcs/runtests.jl
index b428aed11..0eaf2d09a 100644
--- a/test/mcs/runtests.jl
+++ b/test/mcs/runtests.jl
@@ -14,4 +14,7 @@ using Test
 
     @info("test_reshaping.jl")
     include("test_reshaping.jl")    
+
+    @info("test_payload.jl")
+    include("test_payload.jl")
 end
diff --git a/test/mcs/test_payload.jl b/test/mcs/test_payload.jl
new file mode 100644
index 000000000..46b35cb96
--- /dev/null
+++ b/test/mcs/test_payload.jl
@@ -0,0 +1,28 @@
+using Mimi
+using Test
+  
+@defcomp c begin
+end
+
+m = Model()
+set_dimension!(m, :time, 1:2)
+add_comp!(m, c)
+
+function post_trial(sim_inst::SimulationInstance, trialnum::Int, ntimesteps::Int, tup::Union{Nothing, Tuple})
+    data = Mimi.payload(sim_inst)
+    data[trialnum] = trialnum
+end
+
+sim_def = @defsim begin
+end
+
+trials = 10
+
+original_payload = zeros(trials)
+Mimi.set_payload!(sim_def, original_payload)
+
+sim_inst = run(sim_def, m, trials, post_trial_func = post_trial)
+
+@test Mimi.payload(sim_def) == original_payload   # in the original defintion, it's still zeros
+@test Mimi.payload(sim_inst) == collect(1:trials) # in the instance, it's now 1:10
+@test Mimi.payload(sim_inst.sim_def) == original_payload  # the definition stored in the instance still holds the unmodified payload object
\ No newline at end of file