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 Angstrom. angle(O-ring... H-O) congruent to 163 degrees, and angle(ring-O-ring... H) congruent to 103 degrees, 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.