Crack-free carbon molecular sieve membrane supported on a macroporous substrate was formed by coating a layer of poly(furfuryl alcohol) followed by controlled pyrolysis. Molecular sieving membranes of desired thicknesses can be formed by repeating this procedure. Steady-state diffusion fluxes of single-component and binary mixtures of CH4/C2H6 through the membrane were measured. Although large overall concentration gradients were used (to achieve accuracy), concentration-dependent diffusivities were obtained through integral analyses of the flux data. From the concentration dependence of the single-component diffusivities, it is possible to predict binary diffusivities using a simple theory developed by the authors. Good agreement was obtained between theoretical predictions and experimental data for binary diffusion.