In　this　work,　the　reaction　mechanism　of　photocatalytic　oxidation　of　sacrificial　ethanol　during　water-splitting　process　by　titanium　dioxide　(TiO2)　has　been　studied.　The　pure　rutile　TiO2　or　mixed-phase　structure　titania　(P25)　was　employed　as　the　typical　photocatalyst　in　ethanol　oxidation.　The　as-obtained　results　showed　that　the　formation　of　2,3-butanediol　over　TiO2　in　heterogeneous　systems　is　mainly　due　to　the　photochemical　reaction　proceeded　between　acetaldehyde　molecule　and　ethanol　molecule　instead　of　the　direct　coupling　of　α-hydroxyethyl　radicals.　This　is　different　from　the　early　work　claimed　that　the　fundamental　process　to　produce　2,3-butanediol　is　based　on　the　direct　coupling　of　α-hydroxyethyl　radicals　generated　by　TiO2　oxidation.　The　photochemical　reaction　between　acetaldehyde　molecule　and　ethanol　molecule　to　form　2,3-butanediol　can　also　occur　when　the　concentration　of　the　solid　catalyst　was　reduced　to　certain　degree　if　using　P25　as　catalyst　in　heterogeneous　model,　and　the　selectivity　of　2,3-butanediol　would　change　from　ca.　60%　to　0%　when　enlarging　the　concentration　of　P25　step　by　step.　However,　the　selectivity　of　2,3-butanediol　is　relatively　invariable　when　the　concentration　of　catalyst　was　changed　if　using　rutile　as　photocatalyst.　We　thought　that　the　distinct　diffusing　behaviors　for　mobile　·OHf　and　surface　bound　·OHs　generated　on　different　titania　can　explain　the　varied　selectivity　when　the　solid　concentration　of　TiO2　changed.　The　generation　and　diffusion　of　·OH　from　the　surface　of　P25　(80%　anatase)　to　bulk　solution　is　a　key　process　to　inhibit　the　direct　coupling　of　α-hydroxyethyl　radicals　to　produce　acetaldehyde　or　further　overoxidation　products,　and　the　reaction　zone　of　·OHf　depends　on　the　concentration　of　P25.　For　the　case　of　rutile　TiO2　promoted　reaction,　the　lack　of　mobile　·OHf　on　rutile　TiO2makes　the　photochemical　reaction　between　acetaldehyde　molecule　and　ethanol　molecule　more　facile　to　occur　in　bulk　solution　since　the　surface　bound　·OHs　can　only　have　chance　to　attack　the　surface　adsorbed　substrates.　This　may　be　an　important　reason　to　explain　why　the　selectivity　of　2,3-butanediol　in　ethanol　oxidation　was　not　influenced　significantly　by　the　variation　of　rutile　TiO2　concentration.　All　the　results　regarding　ethanol　transformation　during　photocatalytic　process　achieved　here　cast　some　light　on　the　mechanistic　understanding　of　the　reactions　proceeded　on　the　surface　of　solid　catalyst　in　heterogeneous　model　and　in　the　bulk　solution　when　both　catalytic　step　and　photochemical　step　existed　simultaneously.　©　2017　Shanghai　Institute　of　Organic　Chemistry,　Chinese　Academy　of　Sciences.