Active　species　such　as　holes,　electrons,　hydroxyl　radicals　(•OH),　and　superoxide　radicals　(O2•-)　involved　in　the　photodegradation　process　of　methyl　orange　(MO)　over　TiO2　photocatalyst　were　detected　by　several　techniques.　Using　different　types　of　active　species　scavengers,　the　results　showed　that　the　MO　oxidation　was　driven　mainly　by　the　participation　of　O2•-,　holes　and　•OH　radicals.　Characterized　by　the　liquid　chromatography/mass　spectrometry,　the　transversion　of　the　degradation　products　with　the　light　irradiation　time　was　first　analyzed.　Combined　with　the　measurement　of　oxidation　reduction　potential,　dissolved　oxygen,　conductivity,　and　pH　values,　the　degradation　process　of　MO　on　TiO2　under　the　effect　of　the　active　species　was　revealed.　This　was　the　first　time　that　electrodes　were　introduced　to　track　the　degradation　process　in　situ,　and　these　parameters　would　be　helpful　to　explain　the　degradation　processes　of　other　organic　pollutants.　©　2012　American　Chemical　Society.