The radical complex OH-H 2O has been observed by rotational spectroscopy. Spectra for 16OH- 16OH 2 and 18OH- 18OH 2 have been analyzed using a two-state model which accounts for nuclear motion on both the 2A′ and 2A″ potential surfaces. Partial quenching of the OH orbital angular momentum dramatically affects the rotational spectra, and the 2A′- 2A″ energy separation, ρ, is determined to be -146.50744(42) cm -1. The ground state of the complex has ∼86% 2A′ character and the vibrationally averaged OH-OH 2 hydrogen bond distance is 1.952 Å. The magnetic hyperfine constants for the OH proton in the complex are significantly altered from monomer values.