A theoretical ab initio study of the decomposition of ethene and 2-butene episulfones was carried out by means of SCF, MP2, and MP4 methods. Both decompositions occur stereoespecifically through a nonlinear mechanism which makes possible the electronic rearrangement necessary to produce an olefin and SO2. The decomposition of cis-2-butene episulfone is faster than that of trans-2-butene episulfone. The present theoretical results suggest that this "cis effect" is, at least partly, due to the predominance of attractive dispersion forces over repulsion between the two alkyl groups at the transition state.