The photooxidation of chemisorbed CO on TiO2(110) was investigated. Molecular O-2, chemisorbed at 105 K at anion vacancy defect sites on TiO2(110), is shown to undergo excitation leading to either photodesorption of O-2 or photooxidation of coadsorbed CO to produce CO2. The yield of photoproduced CO2 and photodesorbed O-2 follows the same excitation curve versus photon energy with a threshold at the TiO2 band gap, 3.1 eV. The decay time for photoproduced CO2 is essentially identical with the characteristic decay time for photodesorbed alpha-O-2, corresponding to a photodesorption cross section = 8 x 10(-17) cm(2) for 3.94 eV photons. This indicates that both O-2 photodesorption and CO2 photoproduction are related to the same alpha-O-2 species. Compared to O-2 photodesorption, CO2 photoproduction is a minor process, Lattice oxygen in TiO2 is not chemically involved in CO2 formation during ultraviolet irradiation.