Several α-mercaptoacetic and related acids were synthesized and the rates of the amine-catalysed decarboxylation were measured. In the carbanion-forming reaction, the α-mercapto group exhibits a dramatic rate-enhancement, due to the 3d-orbital resonance effect of the divalent sulphide group. For example, the rate of the decarboxylation of diphenylmercaptoacetic acid was 8·8 × 103 fold greater than that