The location of the reaction zone in the catalytic membrane contactor was determined for two-phase reaction system. As a model reaction oxidation of formic acid by Pt catalyst deposited in the ceramic membrane filtration layer was used. It was determined from gas-liquid mass transport experiments that at gas side overpressures from 0.5 to 0.9 bar the liquid was displaced from membrane support layer and that the tortuosity factor of membrane filtration layer is 3.1. It was estimated on the basis Of CO2 fluxes that the reaction zone is close to gas-liquid interface and that the locus of the reaction zone matches with O-2 penetration depth calculated on the basis of formic acid disappearance rate. It was also shown that partition coefficient of formic acid in the membrane filtration layer is higher than 1, otherwise formic acid could not migrate to the reaction zone. Only a small fraction of the catalyst was involved in the reaction. The apparent activation energy was estimated to be around 44 kJ/mol. (c) 2005 Elsevier B.V. All rights reserved.