The hydroxide ion (OH-) has an unusually high mobility in water, comparable to that of the proton. However, a consensus view of the OH- mobility mechanism and its solvation structure has yet to emerge. In addition, X-ray and spectroscopic experiments reveal significant changes in the structural and dynamical properties of water in the presence of OH-. To gain insight into these questions, we have carried out Car−Parrinello molecular dynamics (CPMD) calculations for aqueous NaOH and KOH solutions under ambient conditions over a wide range of concentrations. These simulations are able to reproduce many puzzling phenomena, in particular, the loss of tetrahedral coordination of water (interpreted from a recent neutron diffraction with isotopic substitution experiment) and the appearance of new spectroscopic features at high concentrations. Furthermore, it is demonstrated that the observed behavior is a result of the formation of a variety of compact hydroxide−water complexes. The distribution of these complex...