In　this　study,　the　oxidation　behaviour　of　alloy　617,　alloy　740,　alloy　HR6W　and　Sanicro　25　was　investigated　in　SCW　and　vapour　environment　at　650　degrees　C　for　320　h.　The　weight　gain　and　the　thickness　of　the　oxide　films　of　samples　after　exposure　in　SCW　are　higher　than　that　in　vapour.　Inner　oxidation　phenomena　were　observed　on　alloy　HR6W　and　Sanicro　25,　which　have　higher　content　of　Fe.　The　structure　of　the　oxide　films　formed　on　alloy　617,　alloy　740,　alloy　HR6W　and　Sanicro　25　after　exposure　in　SCW　at　650　degrees　C　for　320　h　is　composed　by　a　Fe-rich　(mainly　hematite)　outer　layer　and　a　Cr-rich　inner　layer.　The　thickness　of　the　outer　Fe-rich　layer　is　related　with　the　Fe　content　in　alloy　matrix.　Oxide　films　spallation　phenomena　were　found　on　alloy　HR6W　after　exposure　in　SCW.