Previous studies of the microwave rotational spectra of gauche CH 2 DCH 2 OH have been extended for the torsional ground state and new studies are reported for the first excited states of the -OH torsion. The extended studies for the ground state hydroxyl gauche, methyl symmetric conformation made it possible to determine the product of inertia coefficients D and E as well as the rigid rotor A , B , and C and the centrifugal distortion coefficients D J and D JK . Likewise for the hydroxyl gauche, methyl asymmetric conformations I and II in the ground state rotational coefficients and selected centrifugal distortion coefficients have been determined. Rotational coefficients B and C and centrifugal distortion coefficients D J and D JK have been determined for all of the excited states. A significant result of the spectroscopic search was that c -dipole transitions were not observed within the range of our spectrometer for either the methyl symmetric or methyl asymmetric conformations in the excited states. For the methyl asymmetric conformation, this means that the tunnelling energy for the excited state has overcome the torsional potential energy term that suppressed the pure hydroxyl tunnelling, that localizes the molecule into conformations I and II for the ground state. The very small tunnelling energy within conformations I and II has been predicted. The role of internal rotation -overall rotation Coriolis coupling including denominator corrections from the rotational energy for hydroxyl gauche, methyl symmetric CH 2 DCH 2 OH is shown in the Appendix to contribute to the effective rotational coefficients.