As a rule, concentration polarization in external fluid boundary layers undercuts selectivity in membrane-based two-component separations, because transport of both components in a binary fluid mixture is governed by the same mutual diffusion coefficient. The same is not generally true of multicomponent boundary layer diffusion; e.g., counterdiffusion of gas C retards the transport of heavier gas B more than that of lighter gas A. We have previously reported evidence of this phenomenon in the permeation of hydrogen and ethane in a microporous inorganic membrane. Because pore transport is governed by Knudsen diffusion, the membrane is intrinsically selective to the lighter hydrogen. However, when permeation was sustained by a permeate channel sweep gas, argon, the apparent ratio of H2 and C2H6 permeabilities exceeded the Knudsen value of approximately 4. This was attributed to counterdiffusion of the feed gas mixture and argon in the external boundary layers. Here we present the results of calculations that indicate that when transport of argon is enhanced by a small increase in sweep gas pressure, it can theoretically preferentially retard the flux of heavier ethane to such a great extent that the effective H2:C2H6 permeability ratio approaches infinity.