SAPO-34 membranes on stainless steel, tubular supports separated CO2 from CH4 at feed pressures up to 3.1 MPa. The highest CO2 permeance was 2.4 x 10(-7) mol/(m(2) s Pa) for a 50/50 feed mixture at a pressure drop of 0.14 MPa. For a pressure drop of 3 MPa, the CO2/CH4 separation selectivities at 253 K were 140-150; at lower pressure drops, the highest selectivity was 270. The highest CO2 flux was 21 kg/(m(2) h) at 295 K and a pressure drop of 3 MPa. Separation selectivity decreased as temperature increased because separation was partly based on competitive adsorption. As transmembrane pressure drop increased, both CO2 flux and CO2 permeate concentration increased for a 50/50 mixture. The flux pressure dependence was modeled by Maxwell-Stefan diffusion for mixtures. Methane decreased the CO2 diffusion rate and, thus, decreased the CO2 flux. The CH4 flux was also lower in a mixture because CO2 inhibits CH4 adsorption.