With the increasingly energy and environmental problems, photoelectrochemical (PEC) water splitting has recently attracted a great deal of attention. The slow oxygen evolution reaction (OER) involved in O-H bond breaking and attendant O=O bond formation has restricted the efficiency of PEC water splitting. Herein, we demonstrate for the first time that the Eu2O3 can work as OER catalyst to significantly promote the performance of PEC water splitting. Because of its surface valance transition, the Eu2O3/ZnO shows an obvious water oxidation peak at 1.90 V vs. RHE, with a cathodic shift of around 200 mV to that of ZnO. The Eu2O3/ZnO can also yield a current density of 3.5 times larger than that of ZnO. Furthermore, similar enhancements can be also observed for the TiO2 and BiVO4, confirming the Eu2O3 is a general and efficient OER catalyst for improving the PEC performance. Such OER catalyst of Eu2O3 may bring new opportunities for the development of PEC cells with superior performance. (C) 2015 Elsevier B.V. All rights reserved.