The　systematic　clarification　of　the　photocatalytic　process　in　different　kinds　of　systems　is　significantly　instructive　to　recognize　the　photocatalytic　mechanism　and　design　novel　efficient　photocatalysts.　The　types　of　the　active　species　(such　as　(OH)-O-center　dot　and　O-2(center　dot-))　generated　in　the　photocatalytic　process　are　complicated　and　their　roles　playing　in　the　photocatalysis　remain　obscure　and　controversial.　Aimed　at　these　problems,　the　active　species　in　the　degradation　process　of　methyl　orange　(MO)　were　investigated　by　using　TiO2　and　ZnO　photocatalysts.　Various　surface　analysis　techniques　(NMR,　ESR,　photoluminescence　technique,　etc.)　were　used　to　study　their　mechanisms　in　the　degradation　of　MO　under　UV　light　irradiation.　As　a　result,　it　was　found　that　in　TiO2　system,　(OH)-O-center　dot　was　generated　by　photo-generated　holes;　while　in　ZnO　system,　(OH)-O-center　dot　was　generated　by　photo-generated　holes　and　O-2(center　dot-).　The　larger　numbers　of　the　active　species　and　their　incessant　sources　in　ZnO　system　contributed　to　the　better　activity.　The　conclusions　were　first　found　in　the　investigation　of　the　active　species　between　the　two　catalysts　and　would　provide　certain　theoretical　guidance　in　seeking　the　new　pathway　to　increase　the　concentration　of　the　active　species　and　the　photocatalytic　activity.　(C)　2013　Elsevier　B.V.　All　rights　reserved.