Spin cast and Langmuir-Blodgett (LB) films of atactic PMMA [poly(methyl methacrylate)] were studied at 25 degrees C by FTIR ATR (Fourier transform infrared spectroscopy, attenuated total reflection) spectroscopy before and after annealing at 140 degrees C and after contact with water vapor or liquid. The dichroic ratios of the carbonyl groups revealed that the average molecular orientations of the films remained unchanged upon annealing and also upon exposure to water. However, annealing helped avoiding cracks which appeared in the nonannealed film after contacting liquid water. The absorbances of individual water bands (of the non-hydrogen-bonded monomers, hydrogen-bonded dimers, larger clusters, and associated chains of water) were determined by detailed spectral analysis and compared with a one-dimensional Fickian model. The absorbance data for the annealed spin cast film of thickness of 5.4 mu m showed poor fit to the one-dimensional Fickian diffusion model, but the apparent diffusion coefficient, D-app, ranged from 5 x 10(-10) to 4 x 10(-11) cm(2)/s. The process of film hydration was found to precede the transfer of bulklike water. For the annealed LB and spin cast films of thickness of 0.1 mu m, D-app ranged between 10(-13) and 10(-15) cm(2)/s. It was estimated from ATR carbonyl band intensities that the thin films were about 44% more dense than the thick film, indicating that they had a tighter structure with fewer structural defects or voids. Having more hydrophobic and less rough surface, the LB film showed a longer initial delay in liquid water transport than the spin cast film of the same thickness.