The gas permeability of poly(alkyl acrylate)s was measured as a function of temperature in both the amorphous and crystalline states. Penetrant permeability in the amorphous state increases as the side-chain length becomes longer but shows mixed trends in the crystalline state. A decrease in permeability was observed in the amorphous state when the backbone was stiffened, i.e. polymethacrylate versus polyacrylate. The influence of side-chain length and backbone stiffness on the magnitude of the change in permeability upon traversing the melting point, or permeability switch, is discussed. The effect of main versus side-chain crystallinity on the permeation switch is analyzed. The influence of a compositional change in the amorphous phase upon melting on the gas permeability of side-chain crystalline polymers is addressed.