Nanowire arrays of mixed oxides of Co and Ni freely standing on Ni foam are prepared by a template-free growth method. The effects of Ni content on the morphology, structure and catalyst performance for oxygen evolution reaction are investigated by scanning electron microscopy, X-ray diffraction spectroscopy and electrochemical techniques including cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy. A transformation from nanowire arrays to nanoplate arrays is found with the increase of the atomic ratio of Ni to Co in the preparation solution. The NixCo3-xO4 electrode obtained at 1:1 of Ni:Co in the preparation solution exhibits nanowire array structure and has better catalytic performance for oxygen evolution reaction than other NixCo3-xO4 and Co3O4 electrodes. The catalytic activities of the NixCo3-xO4 and Co3O4 electrodes are correlated with their surface roughness. Superior stability of the NixCo3-xO4 nanowire array electrode is demonstrated by a chronopotentiometric test. The reaction orders with respect to OH- on the NixCo3-xO4 electrode are close to 2 and 1 at low and high overpotentials, respectively. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All tights reserved.