the invention is a method of separating a gas comprising oxygen , nitrogen or carbon dioxide from a mixture of gases containing oxygen , nitrogen or carbon dioxide wherein the method comprises a . contacting the gas mixture with a membrane containing a thin dense discriminating layer comprising a polymer of sulphonated polyether sulphone under conditions such that oxygen , nitrogen or carbon dioxide selectively permeates through the membrane to the other side of the membrane ; and , b . removing the permeated oxygen , nitrogen , or carbon dioxide from the other side of the membrane . the membranes useful in this invention demonstrate good selectivity for oxygen over nitrogen and carbon dioxide over light hydrocarbons and nitrogen over light hydrocarbons . furthermore , the permeabilities of oxygen and carbon dioxide through these membranes are quite good .