Although　both　H　2　and　CO　can　be　thermodynamically　oxidized　by　O　2　over　TiO　2　under　ultraviolet　light　(UV)　irradiation,　it　was　found　that　CO　not　H　2　could　be　oxidized　over　an　anatase　TiO　2　in　this　work.　The　chemisorption　results　of　CO　and　H　2　at　TiO　2　surface　under　UV　irradiation,　investigated　by　a　gas　sensing　testing,　showed　that　CO　adsorption　at　TiO　2　would　cause　the　decrease　of　TiO　2　surface　impedance,　whereas　H　2　adsorption　would　cause　its　increase.　It　is　proposed　that　the　CO　adsorbed　at　TiO　2　donate　electrons　to　TiO　2　(as　a　process　of　CO　pre-oxidation),　resulting　in　its　oxidation.　In　contrast,　the　H　2　adsorbed　at　TiO　2　accept　electrons　from　TiO　2　(as　a　process　of　H　2　pre-reduction),　which　makes　it　difficult　to　be　oxidized.　This　result　indicates　that　the　photocatalytic　oxidation　of　a　reactant　over　TiO　2　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　TiO　2　.　©　2015　Elsevier　B.V.　All　rights　reserved.