SAPO-34 membranes were synthesized by seeding SAPO-34 crystals on porous stainless steel supports. By using combinations of structure directing agents (SDAs) for both seed and membrane preparation, membranes with high fluxes and high selectivities for CO2/CH4 and CO2/H-2 separations were prepared. Membranes prepared with smaller, homogeneous cubic and rectangular crystals (< 1 mu m) displayed superior separation performance.