diff --git a/src/equation_tree/tree.py b/src/equation_tree/tree.py
index f016e48..ed6503d 100644
--- a/src/equation_tree/tree.py
+++ b/src/equation_tree/tree.py
@@ -552,7 +552,7 @@ def n_non_numeric_constants(self):
 
     @property
     def variables_unique(self):
-        return list(set(self.variables))
+        return sorted(list(set(self.variables)))
 
     @property
     def n_variables(self):