The solvation shell of the hydroxyl ion in water is explored experimentally for the first time by using a combination of neutron diffraction with hydrogen isotope substitution and Monte Carlo simulation within the empirical potential structure refinement framework. The data are compatible with the presence of nonplanar hydrogen bonded (H9O5)(-) complexes. The presence of a fifth water molecule in the hydration shell of the hydroxyl ion, weakly hydrogen-bonded to the hydrogen site is also revealed. (H7O4)(-) complexes, which have been suggested by ab initio simulations to promote proton transfer, are not detectable in the present data, implying that, if they are present at all, they can only be formed transiently. (C) 2003 American Institute of Physics.