Transport of water in an epoxy network with high cross-link density was investigated at several water vapor activities by time-resolved FT-IR spectroscopy and gravimetric measurements. The analysis of the infrared spectra provided information about the molecular interactions occurring in the system. In particular, several interaction complexes were identified and their structures were proposed. An estimate was made of the concentration of the various water species present in the system, based on the knowledge of the respective molar absorptivities. An excellent agreement between the spectroscopic and gravimetric determinations of sorbed water was found throughout. The evolution of the different water species was monitored by resolving the complex profile of the water spectrum in the nu(OH) frequency range. This information, coupled with the results of the gravimetric analysis. was used to evaluate the effect of polymer/penetrant H-bonding interactions on the diffusion process of water molecules. Transport of the different water species was found to follow a Fickian behavior characterized by an effective diffusion coefficient which increases with total water concentration.