This paper describes the photoelectrochemical (PEC) behavior of composite semiconductor films containing Degussa P 25 TiO2 particles dispersed in a WO3 matrix. These films (hereafter designated as "WO3-TiO2") were electrosynthesized using occlusion electrodeposition from a bath contained suspended TiO2 particles and peroxytungstate precursor species. One focus of this study was the effect of pre-calcination of the TiO2 particles (prior to their incorporation into the WO3 film matrix) on the PEC behavior of the resultant composite film. The PEC activity and selectivity of the composite films and their dependence on the pre-calcination temperature were studied in aqueous media containing formate ions as additives in some cases. Another focus of this study was the role of the WO3 matrix itself and how the oxide matrix compares with a metallic (e.g., Ni) counterpart. Finally, the photoaction spectra of the WO3-TiO2 composite films were simulated by a modified diffusion model, previously developed by Sodergren et al. [J. Phys. Chem. 98 (1994) 5552] for microporous semiconductor films. Good agreement was found between the model simulations and experimental photoaction spectra. These simulations confirmed the ability of the WO3 component to accept the photogenerated electrons from the TiO2 component in the composite films and thereby improve the quantum yield. (c) 2004 Elsevier B.V. All rights reserved.