Measurements on the pure transport of oxygen, nitrogen, and carbon monoxide in membranes prepared from poly[bisphenol A carbonate-co-4,4'-(3,3,5-trimethylcyclohexylidene) diphenol carbonate] are reported. The isotherms representing the variation of the permeability coefficient with the upstream pressure can be described by assuming that the diffusants are retained in the matrix by Henry and Langmuir mode sorptions. The values of the permeability coefficient of oxygen, nitrogen and carbon monoxide at 35 degrees C and p(o) = 1 atm are 8.75, 1.87, and 2.91 barrers, respectively. The values of the diffusion coefficient in 10(-8) cm(2)/s, measured in the same conditions, are 8.26, 2.81, and 3.08, respectively. These membranes exhibit higher permeability than polycarbonate membranes with the two methyl groups of bisphenol A replaced by other different molecular groups, without a substantial diminution of permselectivity. The permeability and the diffusion coefficients obey Arrhenius behavior. The application of the transition state approach to the simulation of the transport of diatomic molecules in the membranes is described. Good agreement between simulated and experimental results is found by assuming that the polymer chains of the membrane fluctuate around equilibrium positions with a root mean-square displacement Delta approximate to0.3 Angstrom. 2001 American Institute of Physics.