Author Institution: Dept. of Chem., Harvard University; Chemistry Department Pastore Hall, University of Rhode Island; National Institute of Standards and Technology, Molecular Spectroscopy DivisionThe microwave spectrum of the water-CO complex has been observed using both molecular beam electric resonance and Fourier transform microwave spectroscopy. The water is hydrogen bonded to the carbon of the carbon monoxide. A tunneling motion between the two equivalent hydrogen bonded structures gives rise to two states. These states are unambiguously assigned based on hyperfine structure arising from either the proton spin-spin interaction (H2O) or deutcrium quadrupole hyperfine (D2βO). A barrier to the tunneling motion of about 200cmβ1 is obtained from the difference in the dipole moments of the two states using a simple one dimensional tunneling model. Although the water is hydrogen bonded to the CO, the O-H bond of water is tilted away from a linear hydrogen bonded configuration by about 12β. (The direction is such that the lone pairs on the oxygen, rather than the other hydrogen, are brought into the carbon.) Electrostatic models predict a linear hydrogen bonded structure, with a barrier to tunneling of between 400 and 900cmβ1 depending on the choice of distributed multipole model