In-depth characteristic studies with H-2 activity and theoretical calculations were used to reveal the copper oxidation states most effective for photocatalytic hydrogen production when loaded on TiO2. When the copper was originally present as CuO, hotogenerated electrons initially reduced the Cu2+ to Cu+ in preference to proton reduction. The resulting Cu2O then behaved as a secondary photocatalyst on the TiO2 surface acting to improve the hydrogen production rate (1.4 times greater than neat TiO2). When the copper was originally present as Cu-0, an improved hydrogen generation rate was also evident (2.4 times greater than Cu2O/TiO2) and the metallic state was retained over the course of the reaction. In this case, the Cu-0 deposits function as a co-catalyst for proton reduction. The findings reconcile past disagreements associated with this system, demonstrating both Cu+ (following reduction from Cu2+) and Cu-0 are able to photocatalytically generate hydrogen, albeit by distinctly different mechanisms. (C) 2015 Elsevier B.V. All rights reserved.