The TiO2(110) surface, prepared with different densities of anion vacancy defect sites, has been investigated by studies of O-2 photodesorption at 105 K. Two separate O-2 photodesorption processes, alpha(1) and alpha(2), are detected and are postulated to be due to the presence of two different types of defect sites produced by annealing the crystal in vacuum before Oz adsorption. The measured photodesorption cross sections for these two states are Q(alpha 1-O2) = (3.1 +/- 0.2) x 10(-16) cm(2) and Q(alpha 2-O2) = (4.3 +/- 0.3) x 10(-17) cm(2) for 3.96 +/- 0.07 eV photons. Both O-2 states photooxidize CO chemisorbed on the reduced TiO2(110) surface. While the yield of the CO2 photoproduct increases with increasing crystal annealing temperature (and increasing defect coverage), the yield of alpha(1)-O-2 and alpha(2)-O-2 passes through a maximum at an annealing temperature of similar to 600 K.