Ab initio calculations at the MP2 and QCISD levels of theory using Pople's 6-31G(d,p) and Dunning's aug-cc-pVDZ basis sets were carried out to explore the potential energy surfaces corresponding to the C2H2... SO2 complex. A C-s stacked structure was found to be the most stable one in agreement with microwave experiments. The structural information on other dimers and trimers located may be useful to fit a number of unassigned transitions. The lack of tunneling splittings in the microwave spectrum is explained in terms of the high barrier computed for the internal rotation of the C2H2 subunit.