Photocatalytic overall water splitting using Zn-added Ga2O3 modified with Rh2-yCryO3 showed a weaker hydrogen-deuterium isotope effect and a lower apparent activation energy than previously reported electrochemical H-2 evolution reactions. The photocatalytic reaction rates were dominated by bulk processes, rather than surface redox reactions. The isotope effect varied with the amounts of photocatalyst and cocatalyst, and with light intensity, due to varying contributions from surface reactions. (C) 2010 Elsevier B. V. All rights reserved.