The present study focuses on the photocatalytic activity of carbon-doped TiO2 prepared by the sol-gel method. The decomposition of acetaldehyde under visible light illumination showed that modified TiO 2 catalyst had high activity under visible irradiation (> 420 nm), 8 times higher than that of commercial P25 TiO2. Catalysts were characterized by X-Ray powder diffraction (XRD), UV-vis diffuse reflectance spectrophotometer (DRS), X-Ray photoelectron spectroscopy (XPS), and surface area measurement by N2 adsorption-desorption. It was found that elemental carbon was doped both on the surface and inside the bulk of TiO 2. The elemental carbon acted as a photosensitiser. Under visible light irradiation, the excited photosensitiser injected the electron into the conduction band of TiO2 and then the electron was transferred to adsorbed O2, producing superoxide radical anion O2 - which participated in the oxidation of acetaldehyde.