A novel experimental approach, based on in situ FTIR spectroscopy in the transmission mode, has been developed to monitor sorption-desorption behaviour of small molecules in polymer films. This technique, along: with classical gravimetric analysis, has been used to investigate water vapour transport in an epoxy resin. Aim of the investigation was to elucidate the different types of interaction that water molecules form with the macromolecular network and their change as function of the amount of sorbed water. This analysis has been performed at several water vapour activities (0.08, 0.2, 0.4, 0.6, 0.8) at 24 degreesC by using a FTIR cell specifically designed for the in situ monitoring of water sorption in the epoxy film. Particular attention has been paid to realise a very accurate control of water vapour pressure and temperature in the cell. Water sorption kinetics has been also investigated in the same conditions by using an electronic microbalance. Information gathered from the results of both experimental approaches is potentially useful to clarify the plasticising action of sorbed water. To this aim, the amount of water sorbed by itself, is not a reliable predictor of possible physical effects on the matrix, since the plasticising efficiency of water molecules is expected to change with the level of interaction they establish with the polymeric matrix. FTIR based analysis supplies a very useful experimental tool to discriminate among different types of penetrant-polymer molecular interaction. In fact, several spectroscopically distinguishable 'types' of sorbed water molecules have been detected in the case of the analysed water-epoxy system and their evolution as a function of the penetrant concentration has been followed.