Photocatalytic　surface　sites　on　SO42-/　TiO2　were　investigated　by　pyridine　adsorption　and　in　situ　Fourier　transform　infrared　(FT-IR)　spectroscopy.　Results　revealed　that　the　sulfate-modification　not　only　increased　the　number　of　strong　Lewis　acidic　sites,　but　also　induced　a　large　amount　of　strong　Brønsted　acidic　sites　on　the　surface　of　TiO2.　Pyridine　molecules　were　chemically　captured　on　Brønsted　and　Lewis　acidic　sites　on　the　SO42-/TiO2　surface.　These　pyridine　molecules　were　progressively　decomposed　to　final　products　of　CO2　and　H2O　under　actual　photocatalytic　conditions.　The　high　photocatalytic　performance　of　SO42-/TiO2　can　be　explained　by　the　improved　surface　acidities　that　favor　the　adsorption　of　both　oxygen　and　pyridine　molecules.　Moreover,　the　Lewis　acidic　sites　could　react　with　H2O　and　was　then　converted　to　Brønsted　acidic　sites,　leading　to　the　activation　of　the　water.　This　conversion　promoted　the　formation　of　hydroxyl　groups　on　the　catalyst　surface,　which　could　also　contribute　to　the　high　photocatalytic　reactivity　of　SO42-/TiO2.　©　2005　Elsevier　B.V.　All　rights　reserved.