First, water, methanol, ethanol, acetone, and methyl t-butyl ether were used as molecular probes to measure the free volume distribution of a type of polyimide membrane material (HQDEA-DMMDA). The methods were equilibrium swelling and separation membrane technologies. From the Kirchheim theory of free volume distribution, a Gaussian distribution function was determined. The Gaussian distribution function was confirmed with tensile testing and wide-angle X-ray diffraction of the polyimide films. Second, polyimide/Ag3O4 composite hollow-fiber membranes were prepared by dry/wet phase inversion. The separation performances of the composite membranes were characterized with a methanol/methyl t-butyl ether mixture. The change in the separation performances was explained by the free volume distribution function very well. (C) 2004 Wiley Periodicals, Inc.