Gas separation properties of polymer films based on semicrystalline poly(4-methyl-1-pentene) (PMP) with T (g) = 30A degrees C for permanent gases and some lower hydrocarbons have been experimentally studied in the temperature range of -20 to 80A degrees C. Experiments have been performed using the differential permeability technique involving the determination of the diffusion coefficient by the characteristic time and functional scaling-up methods. It has been shown that PMP as a biphasic system may be characterized by one diffusion coefficient that includes the contributions of diffusion in the amorphous and crystalline phases. It has also been shown that despite the glass transition (phase transition) at 30A degrees C, the permeability coefficients of the test gases exponentially increase with gas temperature and the temperature dependence curves do not exhibit an inflection in the glass transition region. On the other hand, the Arrhenius plots of the diffusion coefficients show a bend over the entire glass transition range in PMP, with the activation energy of diffusion decreasing with an increase in temperature. This fact demonstrates the unusual, earlier unknown effect of increasing activation energy of diffusion E (D) for gases below T (g).