Carbon molecular sieve (CMS) membranes have been studied in the past few years as an alternative to both inorganic and polymeric membranes for gas separation under high temperature and pressure conditions. These membranes are made by the pyrolysis of polymeric precursors, and control of their pore size and separation characteristics is accomplished conventionally mainly by choosing the appropriate precursor and by varying the conditions, such as atmosphere, temperature, and duration, of the carbonization procedure. Often, however, the technique does not succeed to consistently provide the tight pore size control required for the separation of important gas pairs, and thus, an additional post-treatment step is needed. In this investigation steam activation was studied as a post-treatment technique in the preparation of CMS membranes. The goal was to adjust the structural characteristics in order to further improve the membrane properties. The impact on separation performance was evaluated based on gas permeation measurements with test gases, such as He, Ar, H-2, CO2, and CH4, and via nitrogen adsorption to determine the membrane pore volume and internal surface area before and after steam treatment. Steam activation was shown to be an effective technique to improve membrane throughput without adversely impacting selectivity. The application of the post-treatment technique for the preparation of membranes with "reverse selectivity" appropriate for the removal of chemical warfare agents from contaminated air streams is briefly discussed as well.