Variable temperature (2105 to 2150 8C) studies of the infrared spectra (3500 -400 cm 21 ) of 1,1-dimethylhydrazine, (CH 3 ) 2 NNH 2 , in liquid krypton have been carried out. No convincing spectral evidence could be found for the trans conformer which is expected to be at least 600 cm 21 less stable than the gauche form. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from MP2/6-31G(d) ab initio calculations. The predicted infrared and Raman spectra are compared to the experimental ones. The adjusted r 0 parameters from MP2/6-311 รพ G(d,p) calculations are compared to those reported from an electron diffraction study. The energy differences between the gauche and trans conformers have been obtained from MP2 ab initio calculations as well as from density functional theory by the B3LYP method calculations from a variety of basis sets. All of these calculations indicate an energy difference of 650-900 cm 21 with the B3LYP calculations predicted the larger values. The potential function governing the conformational interchange has been predicting from both types of calculations and comparisons have been made. The barrier to internal rotation by the independent rotor model of the inner methyl group is predicted to have a value of 1812 cm 21 and that of the outer one of 1662 cm 21 from ab initio MP2/6-31G(d) calculations. These values agree well with the experimentally determined values of 1852 ^16 and 1558 ^12 cm 21 , respectively, from a fit of the torsional transitions with the coupled rotor model. For the coupled rotor model the predicted V 0 33 (sin 3t 0 sin 3t 1 term) value which ranged from 190 to 232 cm 21 is in reasonable agreement with the experimental value of 268 ^3 cm 21 but the predicted V 33 (cos 3t 0 cos 3t 1 term) value of 273 to 2 139 cm 21 is 25% smaller and of the opposite sign of the experimental value of 333 ^22 cm 21 . These theoretical and spectroscopy results are compared to similar quantities of some corresponding molecules.