Abstract The HO −(aq) ion participates in myriad aqueous phase chemical processes of biological and chemical interest. A molecularly valid description of its hydration state, currently poorly understood, is a natural prerequisite to modeling chemical transformations involving HO −(aq). Here it is shown that the statistical mechanical quasi-chemical theory of solutions predicts that HO · [H 2O] 3 − is the dominant inner shell coordination structure for HO −(aq) under standard conditions. Experimental observations and other theoretical calculations are adduced to support this conclusion. Hydration free energies of neutral combinations of simple cations with HO −(aq) are evaluated and agree well with experimental values.