The rate of ion exchange of Ca2+ by H+ in ultrathin organic LB films of calcium stearate was probed by in-situ Fourier transform infrared attenuated total reflection (FTIR-ATR). Calcium stearate was contacted with aqueous solutions of hydrochloric, acetic, or sulfuric acid. Data and calculations for various film thicknesses and film structures indicate that the ion-exchange process was limited by the rate of mass transfer through the film. Heating at 136 degrees C and cooling changed the film microstructure and made the ion exchange process much faster. Mass transfer was at directions both normal to the film/solution interface and laterally, reflecting possible film defects, which either were present or were induced by the ion-exchange process. The acid counterion (Cl-, CH3COO-, or SO42-) had little effect on the exchange dynamics. No accumulation of the counterion in the film was detected. Microstructural IR information showed that calcium stearate films recrystallized and became more ordered upon conversion to steric acid. The results have implications on the possible uses of LB films as ion-exchange materials or sensors.