Transmission infrared spectroscopy was used to follow the uptake of water into Nation 112 (similar to 50 mu m thick) membrane under conditions that enabled detection of vibrational bands for water in different environments inside membrane pores and channels. The evolution of infrared features for interfacial and weakly hydrogen bonded water were followed upon exposure of initially vacuum dried membranes, exchanged by either Na+ or H+, to low humidity atmospheres. The rapid uptake of water into H+ exchanged Nation 112 precluded time resolved spectral measurements. However, the considerably slower timeframe for water incorporation into Na+ exchanged membrane enabled the evolution of different environments for water to be observed. Under approximately 10% relative humidity, the time dependent increases in absorbance for a mode of interfacial water near 3674 cm(-1) and a mode of more bulk-like, weakly hydrogen bonded water at 3525 cm(-1) in Na+ exchanged Nafion 112 could be fit by a pore diffusion model. The results provide a foundation for the application of multivariate analysis techniques to identify different structures that develop in metal cation exchanged Nation during changes in hydration state. (c) 2007 Elsevier Ltd. All rights reserved.