Porous and nanostructured WO3 semiconductor thin films have been used as electrodes for water photo-electrolysis. As water-splitting cells frequently operate in acidic conditions, the aim of this work is to evaluate the effect of proton intercalation of WO3 electrodes on the performance of the photo-electrolysis process. For this, a first study of proton charge and proton discharge electrode's capacity was performed under cathodic and anodic polarization, respectively, at different voltages and times. Protonated electrodes were then exposed to simulated solar radiation in a photo-electrochemical cell (PEC) and characterized with both chronoamperometry and linear sweep voltammetry. The results indicated that proton intercalation reduces WO3 photo-activity; however, the effect is reversible and after enough time full proton discharge is achieved and photoresponse is recovered. The work points out the influence of the WO3 film microstructure on the discharge kinetics and proposes proton intercalation/deintercalation studies as a characterization technique of photoelectrodes. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.