The conversion of CO2 using light energy (CO2 photoreduction) has the potential to produce useful fuels or valuable chemicals while decreasing CO2 emissions from the use of fossil fuels. Identifying the mechanism and the active sites involved in the formation of negatively charged CO2 species on TiO2 surfaces represents a significant advance in our understanding of CO2 photoreduction. To understand the role of the TiO2 surface acting as a photocatalyst mediating CO2 photocatalytic reduction, excited-state ab initio calculations of CO2 adsorbed on clusters from the (010), (101), and (001) anatase surface planes were performed. Both post-Hartree-Fock calculations on small model surface clusters as well as density-functional theory (DFT) calculations on larger clusters indicate that conduction band electrons in irradiated, stoichiometric TiO2 surfaces may not be transferred to CO2. On the other hand, oxygen vacancies may act as the active sites for CO2 photoreduction.