Membranes with high separation factors and permeation values are important for the success of high temperature membrane reactors. Microporous membranes with molecular sieving ability are potential candidates. In situations were the molecular size of the gases to be separated are very near, however, such size exclusion is not possible. The present paper describes a new technique to modify such microporous membranes by making use of special materials with adsorption selectivity to the gases in question. More importantly, the paper points out that the same adsorption selectivity does not always guarantee gas separation. The total adsorbed volume and the number of available sites are also influential in determining the real separation factors. By using the said technique, a membrane system is developed and tested for use in separating propylene from propane with commendable separation and permeation values.