The effect of thermal hysteresis on the polymer chain packing and permeation properties of two 6FDA-based polyimide isomers was investigated. Thermal quenching resulted in a small increase in the fractional free volume of the polyimides with respect to the samples that had been annealed. Quenching from above the glass-transition temperature also resulted in larger increases in the permeabilities for both 6FDA-6FmDA and 6FDA-6FpDA with respect to annealed samples. Meta-connected 6FDA-6FmDA exhibited a larger increase in the permeability after quenching than the para-connected isomer, 6FDA-6FpDA. This larger increase in the permeability for 6FDA-6FmDA may have been due to differences in the effects of the increases in the free volume on the intersegmental resistance to chain motions. Although physical aging over a 3-month period resulted in a reduction in the permeability of quenched samples of 6FDA-6FpDA, the quenched samples maintained higher permeabilities than the annealed samples. (C) 2003 Wiley Periodicals, Inc.