Among the various analysis modes which can be used in FTIR spectroscopy, the internal reflection mode enables us to gain near-surface information on solids or liquids. The interaction between the evanescent field created upon internal reflection of the infrared beam and a sample can be used to monitor the uptake of water molecules by a polymeric film. In this technique, a polymer film of sufficient thickness is applied to a substrate and a special cell is used to study the water diffusion in the polymer films. Spectra were taken automatically at specified time intervals without disturbing either the specimen or the instrument. Results for polystyrene and poly(methyl methac- rylate) films applied to a ZnS substrate are presented to demonstrate the method. The diffusion coefficients of water in these polymers are calculated using the sorption kinet- ics approach, and the diffusion process in each type of polymer is discussed. The effects of the molecular weight and defects in the films on water transport in the polymers are illustrated. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2465-2473, 1997