Langmuir-Blodgett (LB) films of calcium stearate were studied by FTIR before, during, and after contact with water and aqueous solutions of hydrochloric acid. The films retain from 1.5 to 3 wt % water of hydration, and they absorb at least 8 wt % (possibly up to 16 wt %) water, probably in voids, when contacted with bulk liquid water. The effective diffusivity of water in the films was estimated from the data to be in the range from about 10(-13) to 10(-10) cm(2) s(-1), but water transport was probably neither purely diffusive nor one-dimensional. The effects of ion exchange and thermal annealing at 100-140 degrees C on the film microstructure was monitored by detailed analysis of polarized ATR spectre. Conversion of calcium stearate to stearic acid caused film dehydration, recrystallization, and an increase in trans conformations. Certain implications on the barrier, transport, and ion exchange properties of LB films are discussed.