A new class of carbon molecular sieve membranes (CMSMs) has been prepared by carbonization of polyetherimide-coated mesoporous tubular supports. The membranes show higher permeance and better separation factors than other supported CMSMs reported in the literature for the CO2/CH4 and H-2/CH4 binary mixtures as well as for the CO2/H-2/CH4 ternary mixture. CO2/CH4 separation factors as high as 145 for the equimolar binary and 155 for the ternary mixture were obtained with a CO2 permeance about 0.15 (cm(3)/cm(2).psi.min). The corresponding H-2/CH4 separation factors for the equimolar binary and ternary mixtures were 68 and 50, respectively, with a H-2 permeance of 0.13 (cm3/cm2 psi min). The membrane also shows good stability when tested with CO2 and Ar single gases, as well as with an equimolar mixture of CO2/CH4. To study the mechanism of permeation and separation in CMSMs, tests with single gases as well as with binary and ternary mixtures were performed at different temperatures, transmembrane pressure differences, and feed compositions. Elemental analysis, scanning electron microscopy, and gas adsorption were also employed to study the morphology of the resulting membranes. Elemental analysis shows that although the structure consists mostly of carbon, it also still contains oxygen, nitrogen and hydrogen. Scanning electron microscopy of the cross section of the carbonized membrane shows that the carbonized layer lies essentially within the mesoporous gamma-alumina layer, a result also verified by N-2 adsorption analysis at 77 K. The experimental data were compared with simulation results with the same mixtures using a nonequilibrium molecular dynamics method.