Ab initio calculations at different levels of theory [HF, MP2, MP4SDTQ, and QCISD(T)] and using different basis sets (Pople＇s 6-31G**, Dunning-Huzinaga＇s D95**, and Dunning＇s aug-cc-pVDZ) were tarried out to analyze the potential energy hypersurfaces of the cyclopropane sulfur dioxide and ethylene...sulfur dioxide van der Waals complexes. Both the theoretical methods employed and the basis set chosen exert a great influence on the geometry and nature of the stationary points located: The MP2/aug-cc-pVDZ structures are consistent with experimental data from microwave spectroscopy. Binding energies, dipole moments, and rotational constants are in reasonable agreement with the corresponding experimental values. Calculations predict the existence of tunneling pathways between equivalent structures in accord with the experimentally observed splittings in the rotational spectra: The QCISD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ energy barrier in the case of ethylene...sulfur dioxide is 48 cm(-1), in good agreement with the experimental estimate (30 cm(-1)). For the cyclopropane...sulfur dioxide complex where an experimental estimate cannot be done, the computed barrier is 69 cm(-1). Although covalent forces make appreciable contributions to the stabilization of both van der Waals complexes, the electrostatic component of the intermolecular interaction seems to play an especially relevant role in determining the relative orientation of the two subunits in each complex.