Conjugate addition of thiazolidinethiones and oxazolidinethiones to N-crotonylthiazolidinethiones and -oxazolidinethiones was observed in the presence of excess triethylamine in dichloromethane. The addition takes place by the nitrogen of the heterocycle with high diastereoselectivity. It was observed that the stereoselective addition occurs on the anti-s-cis conformation of the N-enoyl sulfur-containing heterocycle.