Sorption and diffusion data were obtained for water vapor in four different polymers: poly (methylmethacrylate) (PMMA), poly (2-hydroxyethylmethacrylate) (PHEMA), poly (N-vinyl-2-pyrrolidone) (PVP) and poly (acrylonitrile) (PAN) at 35 degreesC using a gravimetric sorption method. Highest sorption was for PVP, followed by PHEMA. PMMA and PAN sorbed very little water. All the polymers exhibit a BET type III sorption isotherm; the large upturn at high activity for PVP and PHEMA is probably due to plasticization of the polymers by water vapor. Sorption data were interpreted using Flory-Huggins theory and the Zimm and Lundberg cluster integral.Fickian diffusion is observed for PHEMA. For PVP, the fractional uptake M-t/M-infinity is linear with the square root of the time up to M-t/M-infinity = 0.6 - 0.8 for all water activities a(w), but it shows a clear water sorption overshoot at a(w) = 55.3% and a(w) = 72.1%, probably due to macromolecular relaxation. PMMA sorption kinetics is also characterized by a maximum in the water uptake. The diffusion coefficient increases significantly with water concentration for PVP and PHEMA, weakly for PMMA, but it is independent of concentration for PAN. (C) 2003 Published by Elsevier Ltd.