diff --git a/src/mlpack/methods/decision_tree/decision_tree.hpp b/src/mlpack/methods/decision_tree/decision_tree.hpp
index 79f31c0cff..eacf40ce7b 100644
--- a/src/mlpack/methods/decision_tree/decision_tree.hpp
+++ b/src/mlpack/methods/decision_tree/decision_tree.hpp
@@ -487,7 +487,10 @@ class DecisionTree :
 
   union
   {
+      //! Stores the type of dimension on which the split is done for internal
+      //! nodes of the tree.
       size_t dimensionType;
+      //! Stores the majority class for leaf nodes of the tree.
       size_t majorityClass;
   };
   /**
diff --git a/src/mlpack/methods/decision_tree/decision_tree_regressor.hpp b/src/mlpack/methods/decision_tree/decision_tree_regressor.hpp
index c0754388ba..a4e6231c16 100644
--- a/src/mlpack/methods/decision_tree/decision_tree_regressor.hpp
+++ b/src/mlpack/methods/decision_tree/decision_tree_regressor.hpp
@@ -453,7 +453,9 @@ class DecisionTreeRegressor :
 
   union
   {
+      //! Stores the split point for internal nodes of the tree.
       double splitPoint;
+      //! Stores the prediction value for leaf nodes of the tree.
       double prediction;
   };