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 ) ฯญ 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 .