diff --git a/simgear/scene/material/mat.cxx b/simgear/scene/material/mat.cxx
index 76bb3820..5433a0f2 100644
--- a/simgear/scene/material/mat.cxx
+++ b/simgear/scene/material/mat.cxx
@@ -201,7 +201,7 @@ SGMaterial::load_texture ( int n )
 }
 
 ssgSimpleState *
-SGMaterial::get_state (int n)
+SGMaterial::get_state (int n) const
 {
     if (_status.size() == 0) {
         SG_LOG( SG_GENERAL, SG_WARN, "No state available.");
diff --git a/simgear/scene/material/mat.hxx b/simgear/scene/material/mat.hxx
index 22cbe534..5f14a306 100644
--- a/simgear/scene/material/mat.hxx
+++ b/simgear/scene/material/mat.hxx
@@ -117,7 +117,7 @@ public:
   /**
    * Get the textured state.
    */
-  virtual ssgSimpleState *get_state (int n = -1);
+  virtual ssgSimpleState *get_state (int n = -1) const;
 
 
   /**