The present invention is a method for synthesizing zeolite membranes, particularly a method for synthesizing supported zeolite membranes by wet gel crystallization in a vapor-phase. The method of the present invention is a method for synthesizing zeolite membranes comprising the steps of first coating a support surface with a wet gel precursor sol to form a uniform layer of the precursor sol on the support surface. The wet gel precursor sol comprises a silicate or aluminosilicate species and a template or structure-directing agent. Then, placing the coated support in a timely manner within a closed vessel containing a sufficient volume of a structure-directing agent so to avoid complete vaporization and to maintain a vapor-liquid coexisting state within the closed vessel under increased temperature and autogenous pressure wherein the support is placed above the liquid surface of the structure-directing agent in a distance that ensures no direct contact between the support and the structure-directing agent throughout the synthesis process and placing the support in a timely manner within the closed vessel so to prevent the wet gel precursor sol from absorbing CO.sub.2 from air. Then, thirdly, heating the closed vessel at a sufficient synthesis temperature for a sufficient time to convert the wet gel precursor sol to a polycrystalline zeolite membrane in the vapor-phase of the structure-directing agent within the closed vessel.
 The invention was made with government support under contract no. DE-AC05-960R22464 awarded by the United States Department of Energy to Lockheed Martin Energy Research Corporation; and the government has certain rights in the invention.