Three types of cation exchange membranes loaded with Ag+ ion are used to study the selective transport of ethene gas in ethene/methane gas mixtures across the membranes. The Ag+ ions act as a carrier for selective transport of ethene. The carrier concentration is varied by two methods : (a) partial substitution of Ag+ ions by H+ ions. The H+ ions do not act as a carrier. (b) Introduction of coions (NO3-) and the corresponding counterions (Ag+) into the membrane phase by treatment of the membranes with concentrated aqueous AgNO3 solutions. The functional relation between the rate of transport of ethene and the carrier concentration reflects structural differences between the membrane matrices.