diff --git a/src/pyrecest/utils/cost_matrix_adjustments.py b/src/pyrecest/utils/cost_matrix_adjustments.py
index f2a56cbae..1e1f6f83e 100644
--- a/src/pyrecest/utils/cost_matrix_adjustments.py
+++ b/src/pyrecest/utils/cost_matrix_adjustments.py
@@ -39,6 +39,11 @@ class CostMatrixAdjustmentResult:
     diagnostics: Mapping[str, Any] | None = field(default_factory=dict)
 
     def __post_init__(self) -> None:
+        object.__setattr__(
+            self,
+            "adjusted_cost_matrix",
+            _as_cost_matrix(self.adjusted_cost_matrix).copy(),
+        )
         object.__setattr__(
             self,
             "diagnostics",
diff --git a/tests/test_cost_matrix_adjustment_result_validation.py b/tests/test_cost_matrix_adjustment_result_validation.py
new file mode 100644
index 000000000..679ce365a
--- /dev/null
+++ b/tests/test_cost_matrix_adjustment_result_validation.py
@@ -0,0 +1,34 @@
+import unittest
+
+import numpy as np
+import numpy.testing as npt
+
+from pyrecest.utils.cost_matrix_adjustments import CostMatrixAdjustmentResult
+
+
+class TestCostMatrixAdjustmentResultValidation(unittest.TestCase):
+    def test_result_takes_ownership_of_adjusted_matrix(self):
+        matrix = np.array([[1.0, 2.0]], dtype=float)
+
+        result = CostMatrixAdjustmentResult(matrix)
+        matrix[:] = -7.0
+
+        npt.assert_allclose(result.adjusted_cost_matrix, np.array([[1.0, 2.0]]))
+
+    def test_result_rejects_invalid_direct_construction(self):
+        invalid_matrices = (
+            np.array([1.0, 2.0]),
+            np.array([[np.nan]]),
+            np.array([[-np.inf]]),
+            np.array([[True]]),
+            np.array([[1.0 + 0.0j]]),
+        )
+
+        for matrix in invalid_matrices:
+            with self.subTest(matrix=matrix):
+                with self.assertRaises(ValueError):
+                    CostMatrixAdjustmentResult(matrix)
+
+
+if __name__ == "__main__":
+    unittest.main()