Gas sorption and transport characterization of a new polymer in the polycarbonate family, based on the bisphenol of 3,3,5-trimethylcyclohexane-1-one (BPI) is reported at 35-degrees-C. By comparison with properties of other known polycarbonates, the effects of inhibition of both packing and segmental motion due to the introduction of the bulky substituent in the backbone are elucidated. The T(g) of the material was measured with differential scanning calorimeter (DSC) and was found to be unusually high for a polycarbonate (233-degrees-C). This indicates a successful inhibition of the large-scale segmental mobility of the polymer. Vari-able C-13 NMR analysis indicated that rotation of one phenylene ring has an unusually high (ca. 10 kcal) energy barrier, whereas the other phenylene ring has a more typical rotation profile (barrier < 3 kcal). The density was measured and found to be low (1.107 g/cm3), indicating a high fractional free volume (FFV) for the polymer. Consistent with expectations, the introduction of the bulky-substituted cyclohexane group gave high permeabilities for the various gases tested (N2, O2, He, CH4, CO2) compared to most standard polycarbonates. On the other hand, the permselectivities were typical for standard polycarbonates. The solubility coefficients of all gases were rather high, as expected for a polymer with such an open structure.