Abstract(S‐Mandelic acid, (S)‐lactic acid, and (S)‐2‐hydroxy‐3‐phenylpropionic acid react with thioacetic acid in the presence of the salt of diisopropyl azodicarboxylate and triphenylphosphine to give the α‐(acetylthio)‐substituted products. (S)‐Mandelic acid reacts with predominant retention of configuration whereas (S)‐lactic acid and (S)‐2‐hydroxy‐3‐phenylpropionic acid react predominantly with inversion. The involvement of α‐lactone intermediates is very likely.