An inert membrane has been used to distribute oxygen to a fixed bed of VPO catalyst in the partial oxidation of butane to maleic anhydride. Two different reactor configurations have been used, namely, a standard inert membrane reactor (IMR), in which oxygen permeates inward from the shell side and the catalyst is packed on the tube side, and an outward now inert membrane reactor (OFIMR), in which the catalyst bed is packed on the shell side and oxygen flows outwardly from the tube side. The radial variation of both the oxygen and hydrocarbon concentrations and of the temperature is different in the two types of reactors investigated, and as a consequence a considerably better performance is obtained for the OFIMR.