  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
$\mathrm{HO\cdot[H_2O]_3{}^-}$ 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.
