SAPO-34 membranes and methods for their preparation and use are described. The SAPO-34 membranes are prepared by contacting at least one surface of a porous membrane support with a synthesis gel. The Si/Al ratio of the synthesis gel can be from 0.3 to 0.15. SAPO-34 crystals are optionally applied to the surface of the support prior to synthesis. A layer of SAPO-34 crystals is formed on at least one surface of the support. SAPO-34 crystals may also form in the pores of the support. SAPO-34 membranes of the invention can have improved selectivity for certain gas mixtures, including mixtures of carbon dioxide and methane.