The activation of TiO2 photocatalyst for photocatalysis under the visible light using Acid Red 44 (C10H7N = NC10H3 (SO3Na)(2)OH) is described. Adjustment of the pH enhanced the photocatalytic activation of TiO2 in the presence of visible light. This confirms that the adsorption of a dye on TiO2 surface is an important factor in dye-photosensitization. The differences in the photocatalytic activation mechanism under visible irradiated conditions with that of UV irradiated condition are proposed. The dye-sensitized photocatalysis under visible light was applied to the decomposition of phenol, is a toxic chemical used in industry and frequently discharged into water. (C) 2003 Elsevier B.V. All rights reserved.