The gas transport properties of some polyarylates, polysulfones, 3,3',4,4'-biphenyltetracarboxylicdianhydride (BPDA)- and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)-based polyimides were correlated to their respective dynamic-mechanical gamma relaxation temperatures, T-gamma. The CO2 and O-2 permeability coefficients (P) at 35 degreesC increased with a decrease in T-gamma, i.e. an increase in the localized chain mobility. A linear relationship between In P and T-gamma was observed, based on which a model for prediction of gas permeability was proposed. Within the In P versus T-gamma plot, best-fit curves for each polymer class resulted in a collection of parallel straight lines. For the polyarylates and polyimides, permeability correlations with T-gamma were better or comparable to those obtained by correlations with fractional free volume, V-f. In fact, polyimide permeabilities exhibited no correlation at all with V-f. With the exception of polysulfones, no significant correlations were observed between T-gamma and V-f. Thus, the correlation between P and T-gamma is not a simple consequence of correlations between P versus 1/V-f and T-gamma versus V-f.