The distribution of water between toluene and the ionic cores of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and those of block ionomer reverse micelles was evaluated by H-1 chemical shift measurements of. water at different temperatures. The block ionomer reverse micelles investigated were composed of a nonionic polystyrene (PS) block attached to an ionic block consisting of either poly(sodium methacrylate) (PMANa), poly(sodium acrylate) (PANa), poly(cesium acrylate) (PACs), or poly((4-vinylpyridinium)methyl iodide) (P4VPMeI). The water content is described by the ratio R, defined as the molar ratio of water either to the number of moles of surfactant for AOT or to the number of moles of ionic repeat units for the block ionomers. It was found that for R = 6 and at 25 degrees C, the distribution coefficient of water (K) decreases in the following order for the different ionic groups in the micelle cores : COO-Cs+ > SO3-Na+ approximate to COO-Na+ much greater than N-py(+)(MeI-, where N-py(+)(MeI- represents pyridine quarternized with methyl iodide. This trend is explained by the stronger interaction of water with the anionic reverse micelle systems compared to that with the cationic reverse micelle system. The distribution of water is therefore governed not only by the interactions between water and the solvent but also by the interactions between water and the micelle core, which contribute significantly. The thermodynamics for the transfer process were determined from the temperature dependence of K. An entropy-enthalpy compensation appeared to exist in the water transfer process. For AOT and PS-b-PACs, the enthalpy Delta H-t degrees and entropyz Delta S-t degrees of transfer were similar to those calculated for the transfer of water fi om a toluene phase to a bulk water phase. For PS-b-PANa, PS-b-PMANa, and PS-b-P4VPMeI, Delta H-t degrees and Delta S-t degrees were found to be more negative. This result is attributed to the stronger interaction energy and greater amount of ordering of the water in the block copolymer cores.