The interaction of water with NaCl nanoparticles at supercritical conditions has been studied by molecular dynamics (MD) simulation. During the nanoparticle nucleation process, water is not only physically adsorbed to the periphery of the NaCl cluster but also exists in a confined state within subsurface regions for several picoseconds. Electrostatic fields generated by the coalescing ions are shown to be on the order of 10(10) V/m, which can drive the hydrolysis of confined water Molecules. A molecular mechanism for the sodium chloride hydrolysis reaction at supercritical conditions is suggested. It involves proton transfer from water to chloride ions, followed by hydrolysis product partitioning. We provide MD results displaying the subsurface hydroxide localization in amorphous NaCl clusters, as well as the partitioning of the HCl into the supercritical water phase.