The free jet millimeter wave spectra of the 1:1 complex between pyridazine and three isotopomers of water (H2O, D2O, and (H2O)-O-18) have been assigned. The derived moments of inertia are consistent with a planar structure of the adduct in which one hydrogen of the water molecule is bound to the nitrogen of the aromatic ring, and the "free" water hydrogen is entgegen to the ring. Information on the motions of water with respect to the ring is obtained.