Cobalt manganese oxides are prepared using a co-precipitation procedure and studied for the conversion of synthesis gas to light olefins. In particular, the effect of a range of preparation variables is investigated in detail. The variables investigated include the precipitate ageing time, pH and temperature of precipitation, the [Co]/[Mn] ratio of the precipitation solution and also reactor conditions such as reaction temperature and H-2/CO molar feed ratio. The effect of different supports on the activity and selectivity of catalysts are also studied. The optimum preparation conditions are identified with respect to catalytic performance for the conversion of synthesis gas to ethylene and propylene. The results are interpreted in terms of the structure of the active catalyst. Generally it has been concluded that catalysts containing cobalt manganese mixed phases are found to be the most active. (c) 2006 Elsevier B.V. All rights reserved.