The diffusion of water into polyimide films was studied by in situ FTIR spectroscopy using several methods of spectral data analysis, namely, difference spectroscopy, least-squares curve fitting, 2D correlation spectroscopy. and normal coordinate analysis. The results gave an insight into the molecular mechanism of diffusion in terms of number and population of penetrant species present in the system and with respect to the nature of the molecular aggregates. In particular, two water species were identified and quantified. i.e., H2O molecules interacting with the carbonyl groups of the polyimide and self-associated water. An enthalpy of formation of -0.9 kcal mol(-1) was estimated for the H2O-polyimide interaction, which points to a relatively weak H-bonding tendency of the imide carbonyls. Finally, the infrared spectrum of the H2O-imide aggregate was calculated by a quantum mechanic (QM) model chemistry to rationalize the effects observed in the spectrum of the water saturated films. The results of the computation were in good agreement with the experiment, confirming the predictive capabilities of the chosen QM method and supporting the proposed molecular structure of the H-bonding aggregate.