The sorption and transport of water vapor in five dense polyimide membranes were studied by thermogravimetry. The sorption isotherms of water vapor in the polyimides could be successfully interpreted by both the dualmode sorption model and the Guggenheim-Anderson-de Boer equation. The water vapor diffusion behavior was found to be nearly Fickian at higher water vapor activities, whereas non-Fickian diffusion was observed at lower water activities. The phenomena could be well described by the mechanism of combined Fickian and time-dependent diffusion. The diffusion coefficient and water vapor uptake in the polyimides were strongly dependent on the polymer molecular structure. Except for the polymide prepared from 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride and 1,3-bis(4-aminophenoxy) benzene, the permeability of water vapor in the dense polyimide membranes predicted from the sorption measurement at 30degreesC corresponded well with the water vapor permeability measured at 85degreesC. Among the polyimides studied, pyromellitic dianhydride-4,4'-diaminophenylsulfone (50 mol%)/4,4'-oxydianiline (50 mol%) showed both high water sorption and diffusion and, therefore, high water vapor permeability, which for vapor permeation membranes is necessary for the separation of water vapor from gas streams. (C) 2003 Wiley Periodicals, Inc.