Although　both　H2　and　CO　can　be　thermodynamically　oxidized　by　O2　over　TiO2　under　ultraviolet　light　(UV)　irradiation,　it　was　found　that　CO　not　H2　could　be　oxidized　over　an　anatase　TiO2　in　this　work.　The　chemisorption　results　of　CO　and　H2　at　TiO2　surface　under　UV　irradiation,　investigated　by　a　gas　sensing　testing,　showed　that　CO　adsorption　at　TiO2　would　cause　the　decrease　of　TiO2　surface　impedance,　whereas　H2　adsorption　would　cause　its　increase.　It　is　proposed　that　the　CO　adsorbed　at　TiO2　donate　electrons　to　TiO2　(as　a　process　of　CO　pre-oxidation),　resulting　in　its　oxidation.　In　contrast,　the　H2　adsorbed　at　TiO2　accept　electrons　from　TiO2　(as　a　process　of　H2　pre-reduction),　which　makes　it　difficult　to　be　oxidized.　This　result　indicates　that　the　photocatalytic　oxidation　of　a　reactant　over　TiO2　not　only　depends　on　the　formation　of　the　photo-generated　carriers　and　the　subsequent　activated　oxidizing　species,　but　maybe　also　depends　on　the　electron　transfer　behavior　at　the　interface　of　the　adsorbed　reactant　and　TiO2.　©　2015　Elsevier　B.V.　All　rights　reserved.