[en] The cyclization-induced rearrangement mechanism proposed by Overman to account for nucleophilic catalysis of the thio-Claisen rearrangement has been tested by application of two criteria, viz., the secondary kinetic deuterium isotope effect at the β (side chain) carbon in phenyl allyl sulfide and the substituent rate effect. The results (k/sub H/k/sub D/ = 1.05 and log k/sub xp/k/sub H/ = 0.25 sigma⁺) do not support the mechanism. Instead, they can be construed to support the previously validated mechanism of nucleophilic triggering of sigmatropic rearrangement