Generally, the bimolecular nucleophilic substitution at the saturated carbon (SN2) and the mechanistically similar substitution at the vinyl carbon (SN2-Vin) cannot be quantitatively compared, because of the many interfering steric and electronic factors. The cis and trans rearrangements of trans c-2,t-3-di-tert-butyl-r-1-methylthiiranium ion 5a into thietanium ions 6a and 6b and that of 2,3-di-tert-butyl-1-methylthiirenium ion 7 into thietium ion 8 can be be compared both experimentally and computationally. They occur with intramolecular SN2 and SN2-Vin mechanisms respectively and are both almost exclusively governed by the nucleofugality of the sulfonium leaving group.