The　development　of　a　highly　efficient　photocatalyst　for　oxidizing　and　further　removing　toxic　As(III)　from　the　water　environment　remains　a　big　challenge.　Herein,　mesoporous　Ti0.5Sn0.5O2　(TSO-0.5)　solid-solution　microspheres　are　synthesized　by　a　facile　solvothermal　method　and　followed　by　annealing.　The　mesoporous　TSO-0.5　microspheres　are　systematically　characterized　in　terms　of　the　crystalline　phases,　morphological,　optical,　photoelectric,　and　photocatalytic　properties　using　X-ray　diffraction,　scanning　electron　microscopy,　N2　sorption-desorption,　X-ray　photoelectron　spectroscopy,　UV-vis　diffuse　reflectance　spectroscopy,　and　electrochemical　analysis.　The　characterizations　show　that　TSO-0.5　is　of　well　crystallinity,　hierarchical　mesoporosity,　broad　light　responsive　capacity,　large　surface　area,　and　excellent　photoelectric　features.　Under　irradiation,　the　mesoporous　TSO-0.5　microspheres　exhibit　ultrahigh　photocatalytic　activity　for　methyl　orange　(MO)　degradation　(0.537　min-1·m-2,　∼8　times　that　of　TiO2)　and　As(III)　removal　(up　to　98.4%).　The　photogenerated　holes　are　determined　to　be　the　primary　active　species　in　the　photocatalysis　process　of　MO　degradation　and　As(III)　oxidation.　Based　on　the　results,　a　possible　mechanism　of　photoinduced　charge　transfer　and　subsequent　reaction　processes　over　TSO-0.5　is　proposed.　©　2018　American　Chemical　Society.