Many parameters of polymers exhibit breaks when temperature passes through glass transition. It is also often assumed that fractional free volume (FFV) at the glass transition temperature (T-g) has a standard value (the isofree volume concept). As gas diffusion (D) and permeability (P) coefficients depend on FFV, and mechanism of sorption and permeation is different above and below T-g, a question can be asked if D and P parameters of various gases in polymers have standard values at corresponding T-g, and, if not, how the values of D(T-g) and P(T-g) vary with T-g in different polymers. To examine this problem, two approaches were used: (1) extrapolation to T-g of numerous P and D values measured at ambient temperatures; (2) an analysis of direct data obtained in different polymers at their T-g. In both cases, qualitatively similar results were obtained: the D(T-g) and P(T-g) values increase with growing T-g independently of the nature of gas. Permselectivity P-i(T-g)/P-j(T-g) and selectivity of diffusion D-i(T-g)/D-j(T-g) are reduced when T-g increases. The dependence of the solubility coefficients S(T-g) = D(T-g)/P(T-g) is much weaker than those of D(T-g) and P(T-g). This conclusion was confirmed by the results of direct measurements of S in a wide range of temperature including T-g, for several gas/polymer systems. An analysis of the results of positron annihilation studies of free volume in polymers led to the conclusion that the observed increases in the D(T-g) and P(T-g) values with T-g are caused mainly by thermal activation of diffusion processes at T-g. (C) 2000 John Wiley & Sons, Inc.