Abstract The microwave spectrum of the two possible O-monodeuterated species has been measured in a mixture containing roughly 25% of each of the four possible OH OH , OH OD , OD OH , and OD OD isotopic species. Both spectra follow a semirigid rotor model without effects due to the tunneling of the two hydroxyl protons. Only one conformer has been detected: one oxygen is gauche with respect to the other; one hydroxyl hydrogen participates in the hydrogen bond, while the second one is trans with respect to the CC bond. Precise values for the quartic centrifugal distortion constants have been obtained for both species. A measure of the dipole moment is given.