C-, V-doped TiO2 photocatalysts are prepared by a sol-gel process. These catalysts are highly active for the degradation of acetaldehyde both under visible irradiation (> 420 nm) and in the dark, especially for 2.0% V-containing co-doped TiO2. Characterization results suggest that vanadium ions are introduced both on the surface and into the bulk of TiO2. A free electron, induced by the formation of V5+ in the sublayers of TiO2, is delocalized and promoted into the conduction band by thermal energy and further transferred to O2, generating superoxide radical anion (O2
•-) that is responsible for degradation of acetaldehyde in the dark. The doped elemental carbon increases the visible light sensitivity, the surface area, and the dispersion of vanadium.