This paper deals with the influence of catalytic membrane structure on the way the gas pressure affects the efficiency of a catalytic membrane reactor (CMR). The CMR is an interfacial contactor, used for wet air oxidation, formic acid solution and air being fed separately from both sides of the catalytic membrane. The gas overpressure can shift the gas-liquid interface into the membrane wall, closer to the catalytic zone, and therefore greatly increase the reaction rate. It has been confirmed that this was not an oxygen partial pressure effect. When compared to a conventional slurry reactor, the contactor CMR showed a reaction rate more than three times higher. (c) 2005 Elsevier B.V. All rights reserved.