Water splitting photoelectrocatalytic reaction mechanism on TiO2 nano particulate polycrystalline film has been studied by means of potentiostatic and potentiodynamic measurements in alkaline solutions under UV irradiation peaking at 365 nm. The TiO2 photoelectrode is reduced by scanning to negative potentials and its photoelectrocatalytic activity is enhanced. This is attributed to the formation of defect states within the bandgap created while in negative potential. The defect states can be responsible for the appearance of the two kinetic regions observed in the potentiostatic experiments. These are located within the potential range -0.1 to 0.3 V (region I) and 0.3-0.8 V (region II) vs. RHE, respectively, and can be activated by different states positioned at different energy levels within the bandgap of TiO2. For film thickness <5 mu m region I does not appear and the open circuit voltage (OCV) shifts to more positive values by 0.35 V as compared to thicker electrodes. The splitting of adsorbed water can be realized through the activation of the 'wet electron' state of adsorbed water molecules, which is activated by photo-excited electrons originating from bandgap states positioned closer to the bottom of the conduction band. Based on the OHad reduction peak of the cyclic voltammograms (CV) and the BET surface area of the similar to 5 mu m thick TiO2 electrode, the electrochemical utilization of TiO2 film specific surface area was found to be 100% and the maximum turn over frequency at the saturation photocurrent 0.7 s(-1). (C) 2012 Elsevier B.V. All rights reserved.