A series of copper manganese oxide catalysts of approximate composition Cu1+xMn2-xO4 (x = 0.54 +/- 0.04), which was aged at various times and at different pH values, has been studied by X-ray photoelectron spectroscopy (XPS). In all cases, copper (II) and manganese (IV) are the only oxidation states. The copper/manganese concentration ratio exceeds the bulk value (0.5) at short ageing times, but falls to <0.4 on prolonged ageing. Sodium, which is believed to be a poison for the CO oxidation reaction; for which, the copper manganese oxide catalyst is the most useful, is formed on all samples, however, its concentration decreases significantly with increased ageing time. (C) 2004 Elsevier B.V. All rights reserved.