Continuous SAPO-34 membranes with low concentrations of non-zeolite pores were synthesized by in situ crystallization on porous stainless steel and alpha-Al2O3 supports. These membranes separated CO2 from CH4 with selectivity as high as 67 at 297 K for a CO2 permeance of 1.6 x 10(-7) mol/m(2) s Pa. The membranes are selective for CO2 because CO2 preferentially adsorbs, and it also diffuses faster than CH4. The CO2 and CH4 heats of adsorption were measured on SAPO-34 crystals as 24 +/-0.8 and 16 +/- 0.8 kJ/mol, respectively. Selectivity depended on temperature, feed pressure, and pressure drop. A maximum in CO2 permeance was observed near 275 K for both the single gas and a CO2/CH4 mixture. The CO2/CH4 selectivity decreased as temperature increased from 253 to 473 K. As trans-membrane pressure drop increased, the CO2 flux increased, but the CO2 permeate concentration showed a maximum of 94.7 mol% for a 50/50 feed. Both permeance and selectivity were stable for at least 1 month. (C) 2004 Elsevier B.V. All rights reserved.