It is here demonstrated that proton-intercalation via negative potential biases enhance the rate constant of methylene blue (MB) photo-electrocatalytic de-colorization on n-type tungsten trioxide hydrate (WO3 center dot 0.33H(2)O as prepared by hydrothermal synthesis), rather than the positive potential bias in promoting the charge separation of photo-generated electrons/holes. Due to the generation of W(V) and W(IV) hydroxyl species with smaller band gaps (in comparison with W(VI)) and the electrochemical activation of WO3 center dot 0.33H(2)O, negatively biased proton intercalation into WO3 center dot 0.33H(2)O clearly enhances its photoelectrocatalytic activity for MB de-colorization, including its visible-light photo-electrocatalytic activity. A possible scheme for promoting the photocatalytic activity of WO3 center dot 0.33H(2)O through proton intercalation under constant potential biases is proposed and discussed. The photocatalytic de-colorization rate of MB on UV100-TiO2 is significantly decreased under the same potential biases due to the difficulty in proton-intercalation (C) 2013 Elsevier B.V. All rights reserved.