The supersonic molecular beam (absorption millimeter wave and microwave Fourier transform) spectra of the 1:1 complex between pyrimidine and four isotopomers of water (H2O, HDO, D2O, and (H2O)-O-18) have been assigned. The derived moments of inertia are consistent with a planar (or nearly 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. Only high level ab initio calculations (6-31G**f-MP2-BSSE) account for the coplanarity of the water with the ring molecule. The hydrogen bond stretching force constant and the dissociation energy have been derived to be k(s) = 13.8 Nm(-1) and Delta E = 21.4 kJ/mol, respectively.