Cobalt-manganese binary spinel oxides of high-dispersity are synthesized by thermal decomposition of hydroxycarbonate precursors in the limits 0.33 less than or equal to Mn/Co less than or equal to 2. The mixed oxides are cation-deficient and contain a high surface concentration of non-stoichiometric oxygen. The oxygen-to-metal surface ratios, determined by ESCA are higher than the stoichiometric value for dioxides. The spinel lattice is tetragonally distorted for atomic ratios Mn/Co > 1. The samples synthesized below 400 degreesC are composed from spherical-shaped particles with diameters in the nano-scale range 6-12 nm. Reduction of the binary spinel oxides with hydrogen proceeds in multiple steps that correspond to the reduction steps Co(III) double right arrow CO(II); CO(11) double right arrow Co reduction and Mn(IV), Mn(III) double right arrow Mn(11). In the temperature range 100-1200 degreesC reduction to metallic manganese has not been observed. (C) 2003 Elsevier Science B.V. All rights reserved.