Ca2Nb2O7　(CN)　nanopolyhedra　with　relatively　high　surface　area　and　small　crystallites　have　been　prepared　by　a　microwave-assisted　template-free　hydrothermal　method　for　the　first　time.　The　samples　are　characterized　by　powder　X-ray　diffraction　(XRD),　UV-vis　diffuse　reflectance　spectra　(UV-vis　DRS),　N2-adsorption,　and　Transmission　electron　microscopy　(TEM).　A　comparative　study　on　the　photocatalytic　degradations　of　methyl　orange　(MO)　in　aqueous　solution　and　benzene　in　gas　phase　has　been　carried　out　between　Ca2Nb2O7　nanopolyhedra　and　TiO2.　Results　show　that　the　MO　and　benzene　could　be　effectively　degraded　and　mineralized　over　the　CN　samples.　Due　to　the　higher　photoabsorption　performance　and　concentration　of　OH　radicals,　TiO2　exhibit　a　superior　photocatalytic　activity　for　the　degradation　of　MO　compared　with　the　CN　samples.　However,　the　weaker　redox　ability　of　the　photogenerated　hole-electron　pairs　induces　the　lower　mineralization　ratio　of　MO.　Fourier　transform　infrared　(FT-IR)　analysis　reveals　that　the　photocatalytic　active　sites　of　TiO2　have　been　blocked　by　stable　intermediates,　leading　to　the　deactivation　of　the　photocatalyst.　In　contrast,　although　the　CN　samples　exhibit　a　lower　photocatalytic　degradation　rate　of　MO,　they　show　a　much　higher　mineralization　ratio　of　MO　and　benzene　compared　with　TiO2.　Therefore,　the　CN　samples　maintain　a　clean　surface　during　the　photocatalytic　process,　resulting　in　a　high　and　stable　photocatalytic　degradation　performance.　In　addition,　water　plays　a　key　role　in　the　photocatalytic　degradation　of　organic　pollutants.　©　2012　Elsevier　B.V.