The structure of water in cetyltrimethylammonium bromide (CTABr) reverse micelles in dichloromethane has bees studied as a function of W = [H2O]/[surfactant] by using the IR absorption due to O-H stretching modes in the 3500-3000 cm(-1) range. We restricted our analysis to the region of small amounts of water (0 < W < 20) where the properties of the system depend strongly upon W. The IR spectra can be represented as a sum of contributions from interfacial and bulklike water, with a minor contribution from water in dichloromethane. The fractions of water in the two "regions" within the reverse micelle were evaluated as a function of W. Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in the reverse micelles is strongly inhibited by an increase in W, and first-order rate constants level off at W > 5. The rate effects can be ascribed to an increase ix polarity and hydration of the reaction region as water is added and are related quantitatively to changes in the IR spectrum.