Two forms of the poly(β-benzyl L-aspartate) were prepared: the α form, with hexagonal packing of the α helix (3. 6 residue/turn) and the ω form, with tetragonal packing of the ω helix (4. 0 residue/turn) and intramolecular benzene ring stacking. The α and ω forms were analyzed by infrared absorption spectra, x-ray diffraction, and thermal analysis. The sorption isotherm of carbon dioxide gas for the α and ω forms was measured in the temperature range from 30 to 40°C. The solubility coefficient of the α form is larger than that of the ω form, reflecting the benzene ring stacking in the side chains of the ω form. The permeability coefficient of carbon dioxide gas for the α and ω forms was studied in the temperature range from 0 to 165°C. The permeability coefficient of the α form was larger than that of ω form. It decreased markedly at the α-ω transition. Such behavior is interpreted on the basis that the side chains of the α form are not arranged regularly, while that of the ω form are arranged to form the regular benzene ring stacking. The diffusion coefficient was evaluated in the temperature range from 0 to 50°C on the basis of the solubility coefficient and the permeability coefficient obtained in this study. The difference in diffusion coefficients for the α and ω forms was interpreted in terms of the distance between helices of the α and ω forms and the free volume concept.