A computational investigation of the title reaction involving semistabilized (R = Ph) and stabilized (R = CO2Me) sulfur ylides has been performed using DFT methods including a continuum model of solvent. Our results provide support for the generally accepted mechanism and are in very good agreement with observed cis/trans selectivities. This study shows that betaine formation is nonreversible, and that selectivity is thereby determined at the initial addition step, in the case of semistabilized ylides. Our analysis indicates moreover that addition TS structures are governed by the steric strain induced by the N-sulfonyl group, which favors the transoid approach in the case of syn betaine formation and the cisoid mode of addition in anti TSs. The observed low trans selectivity is accounted for by the favorable Coulombic interactions and stabilization by C-H...O hydrogen bonding allowed in the cisoid anti addition TS. In the case of stabilized ylides, the endothermicity of betaine formation combined with the high barrier to ring closure render the elimination step rate- and selectivity-determining. Accordingly, the low cis selectivity observed in stabilized ylide reactions is explained by the lower steric strain in the elimination step generated by the formation of the cis aziridine (as compared to the trans case).