The refractive index and gas transport properties (i.e., permeability, diffusivity, and solubility) in the 2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)-based polyimides were systematically investigated in terms of their polymer fractional free volumes (FFVs). The permeability and diffusion coefficients of the 6FDA-based polyimide membranes to hydrogen, oxygen, nitrogen, methane, and carbon dioxide were correlated with their FFVs, which were estimated with van Krevelen's group contribution method. Linear correlations were also observed between the gas transport properties and the refractive index of these polyimides. We described FFV as a function of the refractive index based on the Lorentz-Lorenz equation. Linear correlations were observed between their refractive-index-based FFVs and the gas permeability, diffusivity, and solubility coefficients of these 6FDA-based polyimides membranes. However, the FFVs of the 6FDA-based polyimides calculated from refractive index were 1.16-1.37 times larger than their FFV values. This FFV was dependent on the free-volume space and optical factors, such as the refractive index and molar refraction, which affected the electronic structure and the interactions between the gas molecules and the polymer segments. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2794-2803, 2011