An analysis is performed for predicting conversion in membrane reactors with pores both catalytically active and so narrow that intrapore transport is in the Knudsen regime. The analysis is illustrated for A = B + C (e.g., dehydrogenation) stoichiometry, and for the limiting case of very fast, reversible reactions -i. i.e., large Thiele modulus. Thus, local concentrations are calculated from an equilibrium relationship. ’Supra-equilibrium’ conversion is predicted. However, a distinction is drawn between effects attributable to permselective properties of the membrane and those simply required by le Chatelier’s principle. This analysis of conversion in a catalytic membrane reactor allows the influence of intrinsic membrane properties (such as diffusivity differences) to be separated from those of extrinsic factors (such as differences in trans-membrane pressure and dilution of products in a sweep gas which may be flowing on the permeate side of the membrane). Furthermore, external mass transfer resistance, heretofore generally neglected, is examined from a multicomponent diffusion approach.