The oxidation of conjugated polymer on the surface of conjugated polymer/TiO2 composites is inevitable during the photocatalytic reaction. So it is necessary to research the influence of the degree of oxidation of conjugated polymer on the photocatalytic activity of conjugated polymer/TiO2 composites. Poly(3-hexylthiophene) (P3HT) with different oxidation degree was prepared. P3HT/TiO2 composites were synthesized with oxidized P3HT. P3HT and P3HT/TiO2 composites were characterized by FTIR, gel permeation chromatography (GPC), XPS and UV-vis diffuse reflectance spectra. FTIR. GPC and XPS results showed that the P3HT was oxidized to different degree. UV-vis diffuse reflectance spectra indicated that these composites are responsible to visible light. Photocatalytic activity of the P3HT/TiO2 composites was examined by degrading methyl orange solution under visible light irradiation. According to the apparent rate constants of photocatalytic reaction, moderate oxidation degree of P3HT is beneficial for the photocatalytic activity of P3HT/TiO2 composites, while the excessive oxidation of P3HT would decrease the photocatalytic activity of P3HT/TiO2 composites. (C) 2011 Elsevier B.V. All rights reserved.