The physical properties of n-alkyl acrylate copolymers containing two crystallizeable monomers, including thermal characteristics, structure as determined by small angle X-ray scattering, and gas permeability as a function of temperature, were examined in detail and compared to the corresponding homopolymers. The copolymers exhibit co-crystallization and, thus, for a given average side-chain length have comparable melting temperatures as the corresponding homopolymers. For a given side-chain length, the copolymers have somewhat lower heats of fusion than the corresponding homopolymers because of a reduction in crystallite size as revealed by SAXS. This depression in crystallinity is reflected in the permeability data for the copolymers. Poly(n-alkyl acrylates) exhibit a 'jump' in their gas permeability at the T-m of the side-chain lengths that is mainly caused by a switch in the side-chain morphology from crystalline to amorphous upon melting. The depression in crystallinity for the copolymers results in a smaller permeation jump. The jump breadth correlates with the melting endotherms for these polymers as determined by DSC. Ultimately, the melting endotherms for these copolymer systems provide an excellent tool for predicting permeability changes across the melting region. (c) 2005 Elsevier Ltd. All rights reserved.