Gas permeation properties of crosslinked membranes prepared from a series of poly(ethylene oxide-co-epichlorohydrin) (P(EO/EP)) copolymers with different contents of ethylene oxide are determined by using the constant-volume and pressure-increase method. In addition to the chemical composition, the transport properties are related to the main characteristics of copolymers like the glass transition temperature, crystallinity and crosslinking ratio. Permeation measurements of He, H-2, N-2, O-2, CO2 and CH4 Show that the permeabilities are nearly constant up to an EO content of about 75-80 mol%, then increase rapidly up to a maximum around 90 mol% of EO in the copolymers. The same behavior is observed for the diffusion coefficient and the CO2 sorption coefficient. The presence of an optimal EO composition is explained by the competition between crystalline and amorphous EO sequences. The copolymers present very high CO2 permeability and selectivity respect to other permanent gases even in gas mixtures and under high pressures. (C) 2003 Elsevier B.V. All rights reserved.