Organic/inorganic composite films formed by the alternate adsorption of poly(diallyldimethylammonium chloride) and exfoliated sheets of Lap unite RD, a synthetic sheet-silicate mineral, respond dramatically to changes in ambient relative humidity. We have used a chemically selective analytical technique, infrared (IR) spectroscopy, to monitor the sorption and desorption of water vapor by a film over a wide range (similar to 0-87%) of relative humidity. Correlation of the IR response to separate gravimetric measurements indicated that this method provides a quantitative measure of water in the film. The response of the film was stable over a period of at least 2 months.