The (2+2) cycloadditions of SO3 to ethene, propene, and 2-methylpropene were investigated with Hartree-Fock, MP2, QCISD(T), and hybrid Hartree-Fock/density functional theory (HF-DFT) methods. Experimental data support a (2+2) mechanism for these reactions. The thermally allowed (3+2) cycloadditions of SO3 to ethene and propene were also examined. With the exception of MP2 calculations, all levels of theory predict that SO3 reacts by a concerted (2+2) pathway. The transition structure has considerable zwitterionic character, and there is a strong preference for Markovnikov addition. The rearrangements of sulfites to sultones are disfavored. The origin of the large preference for the normally forbidden (2+2) process is attributed to frontier orbital interactions, and is contrasted to the (3+2) cycloaddition mechanism favored by OsO4.