Photocatalytic　oxidation　of　CO　over　vacuum-pretreated　TiO2　is　performed　in　a　series　of　systems　with　the　introduction　of　O-2,　CO,　and　H,　in　different　orders.　The　photocotalytic　oxidation　of　CO　is　dependent　on　the　order　of　introduction　of　O-2,　CO,　or　H,　and　introducing　O-2,　prior　to　CO　promotes　the　oxidation　of　CO.　Moreover,　an　increase　of　reaction　temperature　suppresses　the　oxidation　of　CO,　but　the　preintroduction　of　H,　reduces　this　suppression　effect.　The　results　of　the　chemisorption　of　O-2,　CO,　and　H-2.　at　the　TiO2,　surface　reveal　that　the　adsorbed　O-2,　heals　the　surface　oxygen　vacancy　(SOV)　sites　of　TiO2　while　the　adsorbed　CO　and　H,　promote　the　formation　of　new　SOVs.　It　is　proposed　that　changes　in　the　amounts　of　adsorbed　O-2,　and　SOVs　are　mainly　responsible　for　the　differences　of　CO　conversion　in　different　systems.