The permeabilities of thin films of the alpha-helix-like poly(beta-peptide), poly(alpha-hexyl beta-L-aspartate), to O-2, N-2, and CO2 penetrants were measured by diffusion experiments at temperatures ranging between 20 and 80degreesC. P and D coefficients were found to be higher for CO2 than for the other two gases, and both parameters increased with temperatures for the three gases. Solubilities of N-2 and O-2 appeared to be independent of temperature whereas that of CO2 decreased significantly upon heating. Solubilities of the three penetrants in the polypeptide were calculated by using advanced Monte Carlo methods specifically developed for the simulation of dense polymers. The structure of the polymer was modeled and optimized in a quasi-hexagonal lateral array of 13/4 helices with an average fixed interchain distance of 14.6 Angstrom. Values predicted at low temperature were in satisfactory agreement with experimental data. At high temperatures significant discrepancies between the measured and calculated data were found for N-2 and O-2, which revealed the limitation of the Monte Carlo method to simulate the influence of temperature on the distribution of the unoccupied space.