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 . [H2O](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. (C) 2003 Elsevier B.V. All rights reserved.