The free jet millimeter wave spectra of five isotopomers of the 1:1 complex between ethylene oxide and water have been assigned. The water molecule lies in the plane of symmetry of ethylene oxide perpendicular to the ring; the water hydrogen involved in the hydrogen bond points toward the ring oxygen, while the “free” hydrogen is entgegen to the ring. The H bond parameters are r(O_ring···H) = 1.92 Å, ∠(O_ring···H−O) ≅ 163°, and ∠(ring−O_ring···H) ≅ 103°, respectively. The large differences of the two angles with respect to those of 1,4-dioxane−water and tetrahydropyran−water are mainly due to dipole−dipole interaction energy effects. The MP2 method using a 6-311++G(d,p) is shown to be a reliable approach for this kind of study. An experimental measure of the difference in zero-point energy between the O···D−O−H and the O···H−O−D species is given by the intensity ratio 3/1 of corresponding lines when a sample of water 50% enriched in D is used.