The microwave spectra of hydroquinone and of its O-D mono- and di-deuterated species have been investigated in the frequency range 8-40 GHz. Only the cis rotamer, which possesses a permanent dipole moment, has been observed. For this conformer the rotational and quartic centrifugal distortion constants have been obtained and the dipole moment has been measured. No doubling due to internal rotation between the two equivalent minima of the hydroxyl groups has been resolved.