The favourable effect of adding CO2 to the feed of reactors used for the oxidation of butane to maleic anhydride (MA) was reported previously [Appl. Catal. A 210 (2001) 27 1], although a clear explanation was not given. Also in the present work, a clear increase in the selectivity and yield to MA was observed upon addition Of CO2, using two different membrane reactor configurations. The effect was more marked in the presence of high butane concentrations. From the results presented, it is concluded that CO2 can oxidise the catalyst surface under the conditions used, rendering it more selective.