Nanostructured WO3 thin films were prepd., and photooxidn. of water at such films was studied in a pH 4.68 soln. The cathodic current at potentials <-100 mV vs. a satd. Ag/AgCl electrode was related to the reversible intercalation of H+ and/or Na+. The photocurrent onset was at ∼100 mV, and the satn. photocurrent was at potentials >800 mV. In the range 300-1000 mV, photocurrent increased linearly with the increasing light intensity, indicating that charge carrier generation dominates the photoelectrochem. cell. Under illumination, linear log|i| vs. potential (Tafel) behavior was registered in the range 300-650 mV. Tafel slopes and exchange current densities are reported. The incident photon-to-current efficiency (IPCE) and the quantum yield (Φ) were high, regardless of the incidence of the light (front side, EE, or backside, SE, illumination). Both IPCEEE and IPCESE increased with film thickness. The low wavelength edge of the action spectra was red shifted and moved toward the absorption band edge. Both ΦEE and ΦSE reached a plateau region at shorter wavelength. In the plateau, ΦSE was ∼1 and independent of the film thickness, whereas ΦEE was ∼20% lower and decreasing with increasing film thickness. Adopting a simple diffusion model for the electron transport, the diffusion length of electrons (L) is 6.7 μm for a 5.0-μm thick film. Higher activation energies, EA, were obtained at lower potentials (e.g., 0.60 eV at 200 mV and 0.32 eV at 300 mV). The EA was <0.21 eV in the range 400-700 mV, and decreased further to 0.06 eV at 1000 mV. The variation of EA with potential was explained by the existence of a distribution of electron-trapping states in an energy range around 0.6 V below the conduction band.