The free jet millimeter wave spectra of the 1:1 complex between 1,4-dioxane and four isotopomers of water (H2O, D2O, HDO, and (H2O)-O-18) have been assigned. The water molecule lies in the plane of symmetry of 1,4-dioxane; the water hydrogen involved in the hydrogen bond is axial with respect to the ring, while the "free" hydrogen is entgegen to the ring. The three atoms involved in the hydrogen bond adopt a bent arrangement with a distance O-ring... H of about 1.90 Angstrom and angle(O-ring... H-O) congruent to 163 degrees.