diff --git a/src/OpenMesh/Core/IO/reader/OMReader.cc b/src/OpenMesh/Core/IO/reader/OMReader.cc
index 9de83dfa..5ed81c58 100644
--- a/src/OpenMesh/Core/IO/reader/OMReader.cc
+++ b/src/OpenMesh/Core/IO/reader/OMReader.cc
@@ -291,7 +291,7 @@ bool _OMReader_::supports(const OMFormat::uint8 /* version */) const
 
 //-----------------------------------------------------------------------------
 
-bool _OMReader_::read_binary_vertex_chunk(std::istream &_is, BaseImporter &_bi, Options &_opt, bool _swap) const
+bool _OMReader_::read_binary_vertex_chunk(std::istream &_is, BaseImporter &_bi, const Options &_opt, bool _swap) const
 {
   using OMFormat::Chunk;
 
@@ -447,7 +447,7 @@ bool _OMReader_::read_binary_vertex_chunk(std::istream &_is, BaseImporter &_bi,
 
 //-----------------------------------------------------------------------------
 
-bool _OMReader_::read_binary_face_chunk(std::istream &_is, BaseImporter &_bi, Options &_opt, bool _swap) const
+bool _OMReader_::read_binary_face_chunk(std::istream &_is, BaseImporter &_bi, const Options &_opt, bool _swap) const
 {
   using OMFormat::Chunk;
 
@@ -589,7 +589,7 @@ bool _OMReader_::read_binary_face_chunk(std::istream &_is, BaseImporter &_bi, Op
 
 //-----------------------------------------------------------------------------
 
-bool _OMReader_::read_binary_edge_chunk(std::istream &_is, BaseImporter &_bi, Options &_opt, bool _swap) const
+bool _OMReader_::read_binary_edge_chunk(std::istream &_is, BaseImporter &_bi, const Options &_opt, bool _swap) const
 {
   using OMFormat::Chunk;
 
@@ -641,7 +641,7 @@ bool _OMReader_::read_binary_edge_chunk(std::istream &_is, BaseImporter &_bi, Op
 
 //-----------------------------------------------------------------------------
 
-bool _OMReader_::read_binary_halfedge_chunk(std::istream &_is, BaseImporter &_bi, Options & _opt, bool _swap) const
+bool _OMReader_::read_binary_halfedge_chunk(std::istream &_is, BaseImporter &_bi, const Options & _opt, bool _swap) const
 {
   using OMFormat::Chunk;
 
@@ -749,7 +749,7 @@ bool _OMReader_::read_binary_halfedge_chunk(std::istream &_is, BaseImporter &_bi
 
 //-----------------------------------------------------------------------------
 
-bool _OMReader_::read_binary_mesh_chunk(std::istream &_is, BaseImporter &_bi, Options& _opt , bool _swap) const
+bool _OMReader_::read_binary_mesh_chunk(std::istream &_is, BaseImporter &_bi, const Options& _opt , bool _swap) const
 {
   using OMFormat::Chunk;
 
diff --git a/src/OpenMesh/Core/IO/reader/OMReader.hh b/src/OpenMesh/Core/IO/reader/OMReader.hh
index 97d364a6..ed042f68 100644
--- a/src/OpenMesh/Core/IO/reader/OMReader.hh
+++ b/src/OpenMesh/Core/IO/reader/OMReader.hh
@@ -126,27 +126,27 @@ private:
 
   bool read_binary_vertex_chunk(   std::istream      &_is,
 				   BaseImporter      &_bi,
-				   Options           &_opt,
+                   const Options     &_opt,
 				   bool              _swap) const;
 
   bool read_binary_face_chunk(     std::istream      &_is,
 			           BaseImporter      &_bi,
-			           Options           &_opt,
+                   const Options         &_opt,
 				   bool              _swap) const;
 
   bool read_binary_edge_chunk(     std::istream      &_is,
 			           BaseImporter      &_bi,
-			           Options           &_opt,
+                   const Options           &_opt,
 				   bool              _swap) const;
 
   bool read_binary_halfedge_chunk( std::istream      &_is,
 				   BaseImporter      &_bi,
-				   Options           &_opt,
+                   const Options     &_opt,
 				   bool              _swap) const;
 
   bool read_binary_mesh_chunk(     std::istream      &_is,
 				   BaseImporter      &_bi,
-				   Options           &_opt,
+                   const Options     &_opt,
 				   bool              _swap) const;
 
   size_t restore_binary_custom_data(std::istream& _is,