The n-type semiconductor tungsten oxide is readily dissolved in aqueous solution at pH > 4 and may be problematic in water splitting catalysis. We have reported that a tungsten oxide photoanode prepared by atomic layer deposition can be stabilized with a Mn-oxo compound for efficient photo water splitting at pH 4 and pH 7. However the molecular mechanism of water oxidation reaction in this robust catalytic system is not known. In this work, the mechanism for oxygen and hydrogen production by photo water splitting using Mn-oxo complex/tungsten oxide heteronanostructures was examined under different experimental conditions by X-ray photoelectron spectroscopy as well as gas chromatographic analysis, O-18 isotope measurements, and pH dependence of photocurrent. We found that the Mn(II) species plays an important role in the catalytic cycle of water oxidation in the Mn-oxo oligomer complex/tungsten oxide system and propose a working model of the Mn-oxo oligomer complex/tungsten oxide catalytic system in photo water splitting. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.