AbstractGas permeabilities of a copolymer of acrylonitrile and methyl acrylate were examined at the temperature range of 25° to 80°C. The change of activation energy of gas permeability constant at the glass transition temperature (65°C) was observed with N2, Ar, O2, and CO2, but no change was observed with He. The observation of the change of slope of the Arrhenius plot of gas permeability is discussed as a function of the diffusion constant (D) of a permeant gas at the glass transition temperature (Tg). The change occurs only if the value ofDatTgis smaller than a certain value (i.e., 5 × 10−8), according to the analysis of data appearing in the liter