A theoretical study on a catalytic polymeric dense membrane reactor (CPDMR) is performed. The conversion enhancement over the thermodynamic equilibrium value is studied for an equilibrium gas-phase reaction of the type aA + bB <----> cC + do for three different conditions: Deltan > 0, Deltan = 0 and Deltan < 0, where Deltan = (c + d) - (a + b). For each of these cases, it is studied the influence of the reaction product sorption and diffusion coefficients. The model used considers perfectly mixed flow patterns and isothermal operation in the retentate and permeate sides. It is concluded that the conversion of a reversible reaction can be significantly enhanced when the diffusion coefficients of the products are higher than the reactants' and/or the sorption coefficients are lower. The extension of this enhancement depends on the reaction stoichiometry, global concentration inside the membrane, Thiele modulus and contact time values. It was also observed that it would be preferable to have a conversion enhancement based on higher product diffusion than on lower product sorption coefficients, since this leads to smaller reactor size. Since the performance of a dense membrane reactor depends on both the sorption and diffusion coefficients in a different way, a study of such a system cannot be based only on the permeabilities of the reaction components.