CuWO4 is a medium bandgap (2.3 eV) n-type semiconductor capable of photoelectrochemical water oxidation under applied electrical bias. Here, we show for the first time that suspended microcrystals CuWO4 evolve oxygen photocatalytically under visible illumination from solutions of 0.05 M AgNO3 (10.8 mu mol/hour; AQE of 0.56% at 400 nm) and 0.0002 M FeCl3 (1.5 mu mol/hour). No oxygen is detected with 0.002 M [Fe(CN)(6)](3-) as sacrificial agent. The activity dependence on the redox potential of the acceptors is due to the presence of Cu2+ based electron trap states in CuWO4. According to surface photovoltage spectroscopy and electrochemistry, these states are located on the particle surface, 1.8 eV above the valence band edge of the material. Controlling the chemistry of these states will be key to uses of CuWO4 particles in tandem catalysts for overall water splitting. (C) The Author(s) 2018. Published by ECS.