Abstract By treating β-mercaptopropionic acid with aryl (aralkyl) isothiocyanates in weak alkaline medium, the authors have obtained the corresponding β-(N-aryl (aralkyl)- thiocarbamoylthio)-propionic acids which, heated 2 hours at 120, yield the corresponding 3- aryl (aralkyl)-4-oxo-2-thiono-tetrahydrothiazines.