The scope of this paper is to present a theoretical study of a catalytic polymeric dense membrane reactor (CPDMR) assuming isothermal conditions, plug flow pattern without pressure drop in both retentate and permeate sides, shell side feed and cocurrent operation. The conversion enhancement over the thermodynamic equilibrium value is studied for a gas-phase reaction of the type aA reversible arrow bB, considering two different stoichiometric ratios: Delta n > 0 and Delta n < 0, where Delta n = b - a. For each of these cases, the influence of the relative sorption and diffusivity of the reaction species is studied. It is concluded that the conversion of a reversible, reaction can be significantly enhanced when the diffusivity of the reaction products is higher than that of the reactants and/or the sorption is lower. It is also concluded that, even for equal sorption and diffusion coefficients, the conversion could also be improved for reactions with Delta n not equal 0. The extension of this enhancement depends on the reaction stoichiometry, the overall concentration inside the membrane, the Thiele modulus, and the contact time. (c) 2005 Elsevier B.V. All rights reserved.