Gas sorption in amorphous ethyl and decyl acrylate polymers and semi-crystalline and molten octadecyl and behenyl acrylate polymers were measured as a function of temperature for CH4 and CO2. Diffusion coefficients, calculated based on permeability data for these polymers, are found to increase with increasing side-chain length. The large differences in permeability observed between poly(methyl acrylate) and poly(ethyl acrylate) are described in terms of the diffusion and solubility of gases in these polymers. Anomalous permeation switch behavior of poly(behenyl acrylate) is further analyzed using diffusion switch data. The solubility coefficients for CH4 and CO2 in amorphous poly(alkyl acrylate)s are correlated with alkyl content and extrapolated to the long side-chain limit. The extrapolated values are larger than those for amorphous polyethylene found by extrapolation to zero crystallinity using the traditional two-phase model.