The photo-oxidation of water on the monoclinic P2(1)/nWO(3) (200, 020, and 002) surfaces is investigated using density functional theory calculations, employing the PW91-generalized gradient approximation, and the method developed by Norskov et al. [J. Phys. Chem. B 108, 17886 (2004)] based on the free energy differences between the reaction intermediates. We first relax the bulk material unit cell and then investigate the relative stability of different surface terminations of WO(3) and analyze the overpotential needed for the photoelectrolysis of water. We found that the rate limiting step is the transfer of a proton from the surface adsorbed OH to the electrolyte, and that the computed overpotential for O(2) evolution (1.04 V) is available upon illumination of the surface with visible light.