The photoelectrochemical oxidation of Fe2+ ions on porous nanocrystalline TiO2 electrodes was investigated using in situ electrochemical quartz crystal microbalance (QCM). It was found that the adsorption of Fe2+ ions at the surface of TiO2 electrodes strongly depended on the pH of the solution, and that the saturated amount of adsorbed Fe2+ ions was approximately 4.4 ions nm(-2) at pH 4. The protons produced in photochemical reactions could cause the release of adsorbed Fe2+ ions from TiO2 surface, therefore, the frequency of QCM increased at the initial stage of illumination, during which the direct oxidation of adsorbed Fe 21 ions by photogenerated holes was dominant. When the frequency decreased, the major reaction was the photoelectrochemical deposition of Fe2O3 from the bulk solution via a hydroxyl radical-mediated oxidation pathway, whose efficiency depended on the Fe2+ ion concentration.