WO 3 mesoporous films were prepared using a sol-gel method to serve as the photoanode for water cleavage in a 1 M HClO 4 solution with Pt as the counter electrode. Post-treatment of the as-synthesized WO 3 film with methanol and ethanol vapors, especially with methanol, improves the photocurrent during photoelectrolysis of water. Water and hexane vapors have a deleterious influence on the WO 3 film. Scanning electron microscopy and X-ray diffraction analyses showed that treatment with methanol vapors did not alter the configuration of the nanocrystalline framework or the WO 3 mesoscopic structure. Optical absorption and W L 3 -edge X-ray absorption near-edge structural analyses also revealed that the W-ion electronic and oxidation states remained unchanged after methanol treatment. However, extended X-ray absorption fine structure analysis of the W L 3 -edge showed that the coordination number of W 6+ sites in the WO 3 film significantly increased with the methanol treatment, indicating a corresponding decrease in the defect-state density of the film. The observed increase in the coordination number resulted in a 25% increase in the electron transit rate of the WO 3 film and enhanced solar energy conversion by 32% for the photoelectrolysis of water. We conclude that post-treatment with methanol vapor remedies the defect region in nanocrystalline WO 3 films, and improves the film's electron transport performance in a photoelectrochemical cell.Scheme 1 Processes of charge transport, back transfer, and chemical interaction in a photoelectrochemical cell for water cleavage. The n-type semiconducting photoanode is positively biased for generation of O 2 gas, with corresponding H 2 gas generation on the counter electrode. V CB , V VB , V FB , and E F represent the conduction-band, valence-band, flatband, and Fermi-energy levels of the electrodes.