a method for producing integral , asymmetric membranes for separating mixtures of gases is disclosed . a membrane forming polymer , a solvent for the polymer , and a swelling agent are mixed to form a pourable liquid mixture ; the solvent has a lower boiling point than the other components in the mixture , the polymer is not soluble in the swelling agent , and the swelling agent does not react with the polymer or the solvent . the liquid mixture is spread to form a thin film , the film is exposed to air or an inert gas to form a surface skin by evaporation and is thereafter contacted with a precipitation agent . a preferred integral , asymmetric membrane is produced when the membrane forming polymer is a polyetherimide , the solvent is a halogenated hydrocarbon and the swelling agent is one or more compound selected from the group consisting of an alkyl substituted benzene , an aliphatic carboxylic acid , and a chlorinated hydrocarbon .