Carbogenic molecular sieve membranes supported on macroporous sintered stainless steel flat plates have been synthesized by pyrolysis of a (poly)furfuryl alcohol-acetone solution imbibed within the surface pores of the support. Permeances of H-2, He, Ar, O-2, N-2, and SF6 were measured to determine the permselectivity of these membranes. Experiments performed on flat-plate membranes revealed molecular sieving behavior with the permeance decreasing with increasing molecular size. Separation factors of 2-3 for O-2/N-2 and up to 30 for H-2/N-2 were obtained from single gas permeation experiments at 293 K. Higher temperature experiments demonstrated the activated nature of transport of various molecules. A 1:1 O-2/N-2 mixture was partially separated over a range of pressures at steady state. The lack of pressure dependence of the permeances indicated that shape- and size-selective effects dominated the separation. An EDAX analysis of the flat-plate surface reveals a defect-free CMS layer.