The permeation of supercritical CO2 and SF6 across two different types of microporous membranes has been studied as a function of temperature and feed pressure. A general picture appears where for both species the permeance as a function of the feed pressure has a maximum. For both the polymeric and the silica membranes close to the temperature where the different species become supercritical the maximum in the permeance is observed at the critical pressure. In the supercritical regime the main mechanism for the mass transport is viscous flow, and there is almost no contribution from surface diffusion through the micropores. The permeance of the fluids obtained for different condition can be described by a single mobility constant, which is independent of temperature and pressure. (c) 2006 Elsevier B.V. All rights reserved.