The potential energy surface for the interaction between methanol and sulfur dioxide has been extensively explored by using ab initio methodology at the correlated level [MP2, MP4, QCISD(T)] with Pople's 6-31 G** and Dunning's aug-cc-pVDZ basis sets. Three van der Waals complexes (minima) were located and characterized on the potential energy surface, The structural and energetic parameters of one of them correspond to the ones reported by Kuczkowski and co-workers from microwave spectroscopic studies. Ten transition structures connecting equivalent and inequivalent equilibrium configurations of the van der Waals minima were located and characterized. Two of them are associated to the large amplitude motions responsible for the splittings observed in the rotational spectra: (a) an inversion motion involving both the methanol and sulfur dioxide subunits (mostly the former) and (b) the methyl group internal rotation (torsion). In the first case, theoretical calculations give an estimate of the inversion barrier and provide structural information that, together with the experimental data, may well help to make an experimental prediction of such a barrier. Regarding the methyl group torsional barrier, the theoretical results give support to the idea that the drastic reduction experimentally observed in this barrier height when passing from bare methanol to the methanol...sulfur dioxide complex should be ascribed in part to dynamical effects. (C) 1997 American Institute of Physics.