Spin cast films of poly(hexylmethacrylate azobenzene-sulfone) (PHMA-AS) with a thickness of 7.5 mu m were studied at 25 degrees C by Fourier transform infrared attenuated total reflectance spectroscopy before and after annealing at 140 degrees C, and before and after contact with water vapor or liquid. Spectral analysis of the hydrocarbon and water bands was used to probe the changes in the film microstructure after these treatments. Slight orientation changes were detected in the hydrocarbon region after hydration. No changes were observed, however, in the dichroic ratios of the carbonyl and the chromophore groups of the PHMA-AS films. These groups had average tilt angles of 62 degrees and 69 degrees, respectively, from the surface normal. The addition of side chains consisting of a spacer and a chromophore group to the backbone of PMMA makes the film a better barrier material for water, as it absorbs less water vapor and less liquid water. The density of the sorbed water at 89% relative humidity was estimated to be 0.006 g per cm(3) of polymer. Initially water permeated fast, as non-bulk-like water, through film defects before diffusing more slowly, as bulk-like water, throughout the film. The estimated total density of liquid water absorbed by the film was 0.05 g per cm(3) of polymer. The in-situ transport data suggest transport mechanisms more complex than the one-dimensional Fickian diffusion, evidently due to the complex microporous structure of the films.