The dielectric constant and gas transport properties (i.e., permeability, diffusivity, and solubility) in 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)-based polyimides were systematically investigated in terms of their polymer fractional free volumes (FFVs) at 30 degrees C. The permeability and diffusion coefficients of the 6FDA-based polyimide films to hydrogen, oxygen, nitrogen, methane, and carbon dioxide were correlated with their FFVs estimated using van Krevelen's group contribution method. There appeared, however, small linear correlation coefficients. Linear correlations were also observed between the gas transport properties and dielectric constant of these polyinlides. This study described FFV as a function of the dielectric constant based on the Clausius-Mossotti equation. It was found that the gas permeability and diffusion coefficients of these 6FDA-based polyimide films increased as their dielectric constant-based FFV increased. A better linear relationship was observed between the gas transport properties and the FFV determined from the polymer dielectric constant in comparison to that estimated using the group contribution method. (C) 2007 Wiley Periodicals, Inc.