In the present work, the degradation of methyl orange in water was used to examine the effect of montmorillonite on the photocatalytic activity of TiO2 nanoparticles (P25). Meanwhile, the role which montmorillonite plays in the photocatalysis was explored with UV-visible diffuse reflectance spectra (DRS) and electrophoresis. It is found that montmorillonite can inhibit the photocatalytic decomposition of methyl orange, but the photocatalytic activity of P25 will be enhanced obviously at a certain concentration of montmorillonite. The enhanced photocatalytic activity might be ascribed to the light scattering induced by the montmorillonite particles. Additionally, the effect of montmorillonite on the photocatalytic activity of P25 was studied as functions of pH of the solution, initial concentration of methyl orange, and reaction atmosphere. (C) 2010 Elsevier B.V. All rights reserved.