diff --git a/idaes/core/base/property_base.py b/idaes/core/base/property_base.py
index 128f22beca..d334658f75 100644
--- a/idaes/core/base/property_base.py
+++ b/idaes/core/base/property_base.py
@@ -781,7 +781,13 @@ def __getattr__(self, attr):
             attr: an attribute to create and return. Should be a property
                   component.
         """
-        return build_on_demand(self, attr)
+        try:
+            # try the Pyomo Block's __getattr__ method first which will return
+            # decorators for creating components on the block (e.g. Expression,
+            # Constraint, ...).
+            return super().__getattr__(attr)
+        except AttributeError:
+            return build_on_demand(self, attr)
 
     def calculate_scaling_factors(self):
         super().calculate_scaling_factors()
diff --git a/idaes/core/base/reaction_base.py b/idaes/core/base/reaction_base.py
index fc21c47775..981089346f 100644
--- a/idaes/core/base/reaction_base.py
+++ b/idaes/core/base/reaction_base.py
@@ -361,7 +361,13 @@ def __getattr__(self, attr):
             attr: an attribute to create and return. Should be a property
                   component.
         """
-        return build_on_demand(self, attr)
+        try:
+            # try the Pyomo Block's __getattr__ method first which will return
+            # decorators for creating components on the block (e.g. Expression,
+            # Constraint, ...).
+            return super().__getattr__(attr)
+        except AttributeError:
+            return build_on_demand(self, attr)
 
     def calculate_scaling_factors(self):
         super().calculate_scaling_factors()