The radical complex OH-H2O has been observed by rotational spectroscopy. Spectra for (OH)-O-16-(OH2)-O-16 and (OH)-O-18-(OH2)-O-18 have been analyzed using a two-state model which accounts for nuclear motion on both the (2)A' and (2)A" potential surfaces. Partial quenching of the OH orbital angular momentum dramatically affects the rotational spectra, and the (2)A'-(2)A" energy separation, p, is determined to be -146.50744(42) cm(-1) . The ground state of the complex has similar to86% (2)A' character and the vibrationally averaged OH-OH2, hydrogen bond distance is 1.952 Angstrom. The magnetic hyperfine constants for the OH proton in the complex are significantly altered from monomer values. (C) 2004 Elsevier B.V. All rights reserved.