The far-infrared spectra of dimethyl sulfide (CH3)(2)S and (CD3)(2)S have been determined from ab initio calculations and a number of the weaker bands reassigned to cross sequence transitions between the higher torsional levels. For this purpose, the potential energy surface for the double methyl rotation was calculated with the MP4/6-31G(df,p)/MP2/6-31G(df,p) basis set. The energy values, as well as the kinetic parameters, were fitted to double Fourier expansions as functions of the rotational angles. The torsional solutions were developed on the basis of the symmetry eigenvectors of the G(36) nonrigid group. The torsional bands of the infrared active transitions were found to be clustered into quartets. For most of the bands the torsional splittings were calculated to be too small to be resolved. The intensities of the transitions to the torsional components were found to be roughly proportional to their nuclear spin weights with the G --> G component being the most intense.