SAPO-34 membranes were prepared by in situ crystallization on a-Al2O3 porous supports. The crystal size of the seeds was effectively controlled in the 0.7 to 8.5 mu m range by employing different structure-directing agents. Seeds smaller than 1 mu m produced membranes with CO2/CH4 separation selectivities higher than 170 and unprecedented CO2 permeances as high as similar to 2.0 x 10(-6) mol/m(2).s.pa at 295 K and a feed pressure of 224 kPa. The membranes effectively separated CO2/CH4 mixtures up to 1.7 MPa.