A novel mechanism for the epoxidation of enals with hydrogen peroxide catalyzed by diarylprolinol silyl ether supported by experimental C-13 kinetic isotope effects (KIEs) and density functional theory calculations is presented. Normal C-13 KIEs, measured on both the carbonyl- and beta-carbon atoms of the enal, suggest participation of both carbon atoms in the rate-determining step. Calculations show that the widely accepted iminiumion mechanism does not account for this experimental observation. A syn-S(N)2' substitution mechanism, which avoids formation of a discrete iminium-ion intermediate, emerges as the most likely mechanism based on agreement between experimental and predicted KIEs.