The potentiostatic anodization of metallic tungsten has been investigated in different solvent/electrolyte compositions with the aim of improving the water oxidation ability of the tungsten oxide layer. In the NMF/H2O/NH4F solvent mixture, the anodization leads to highly efficient WO3 photoanodes, which, combining spectral sensitivity, an electrochemically active surface, and improved charge-transfer kinetics, outperform, under simulated solar illumination, most of the reported nanocrystalline substrates produced by anodization in aqueous electrolytes and by solgel methods. The use of such electrodes results in high water electrolysis yields of between 70 and 90% in 1M H2SO4 under a potential bias of 1 V versus SCE and close to 100% in the presence of methanol