Mixed iridium-ruthenium oxide is a promising electrocatalyst for the oxygen evolution reaction. The interaction of the two elements and their contribution to the catalytic activity are of fundamental interest. An iridium-ruthenium oxide catalyst was therefore prepared hydrothermally and characterised by cyclic voltammetry, steady state polarisation measurements, X-ray diffraction and X-ray photoelectron spectroscopy. The catalysts were shown to be solid solutions. Due to significant surface segregation of IrO2 the range of surface compositions was much narrower than the bulk composition-range. The charge-normalised current densities at constant potential for these surface-segregated solid solutions were found to be similar to those obtained at catalysts prepared by physically mixing corresponding ratios of the end-member oxides IrO2 and RuO2. Tafel slopes were in the order of 40 mV dec(-1) for the end members and slightly higher for intermediate compositions. (C) 2012 Elsevier Ltd. All rights reserved.