diff --git a/src/OpenMesh/Core/IO/reader/OMReader.cc b/src/OpenMesh/Core/IO/reader/OMReader.cc
index 5ed81c58..cab6fd42 100644
--- a/src/OpenMesh/Core/IO/reader/OMReader.cc
+++ b/src/OpenMesh/Core/IO/reader/OMReader.cc
@@ -155,7 +155,7 @@ bool _OMReader_::read(std::istream& _is, BaseImporter& _bi, Options& _opt)
 
 //-----------------------------------------------------------------------------
 
-bool _OMReader_::read_ascii(std::istream& /* _is */, BaseImporter& /* _bi */, Options& /* _opt */) const
+bool _OMReader_::read_ascii(std::istream& /* _is */, BaseImporter& /* _bi */, const Options& /* _opt */) const
 {
   // not supported yet!
   return false;
@@ -164,7 +164,7 @@ bool _OMReader_::read_ascii(std::istream& /* _is */, BaseImporter& /* _bi */, Op
 
 //-----------------------------------------------------------------------------
 
-bool _OMReader_::read_binary(std::istream& _is, BaseImporter& _bi, Options& _opt) const
+bool _OMReader_::read_binary(std::istream& _is, BaseImporter& _bi, const Options& _opt) const
 {
   bool swap_required = _opt.check(Options::Swap) || (Endian::local() == Endian::MSB);
 
diff --git a/src/OpenMesh/Core/IO/reader/OMReader.hh b/src/OpenMesh/Core/IO/reader/OMReader.hh
index ed042f68..26801477 100644
--- a/src/OpenMesh/Core/IO/reader/OMReader.hh
+++ b/src/OpenMesh/Core/IO/reader/OMReader.hh
@@ -110,8 +110,8 @@ private:
 
   bool supports( const OMFormat::uint8 version ) const;
 
-  bool read_ascii(std::istream& _is, BaseImporter& _bi, Options& _opt) const;
-  bool read_binary(std::istream& _is, BaseImporter& _bi, Options& _opt) const;
+  bool read_ascii(std::istream& _is, BaseImporter& _bi, const Options& _opt) const;
+  bool read_binary(std::istream& _is, BaseImporter& _bi, const Options& _opt) const;
 
   typedef OMFormat::Header              Header;
   typedef OMFormat::Chunk::Header       ChunkHeader;