Using the electron paramagnetic resonance technique, we have elucidated the multiple roles of water and carbonates in the overall photocatalytic reduction of carbon dioxide to methane over titania nanoparticles. The formation of H atoms (reduction product) and (OH)-O-center dot radicals (oxidation product) from water, and CO3-radical anions (oxidation product) from carbonates, was detected in CO2-saturated titania aqueous dispersion under UV illumination. Additionally, methoxyl, (OCH3)-O-center dot, and methyl, (CH3)-C-center dot, radicals were identified as reaction intermediates. The two-electron, one-proton reaction proposed as an initial step in the reduction of CO2 on the surface of TiO2 is supported by the results of first-principles calculations.