A process is provided for producing alkylthio-substituted carboxylic acids and arylthio-substituted carboxylic acids and their corresponding alkylthio-substituted thioesters and arylthio-substituted thioesters. The process comprises reacting a mercaptan with a lactone in the presence of an acidic catalyst at temperatures of about 100.degree. C. to about 350.degree. C. The process for preparing the thio-substituted carboxylic acids and thio-substituted thioesters may be a continuous process or a batch process.
