2-Amino-4-methylthiazol-3-oxide 5 is reacted with isocyanates and with acid chlorides. Aliphatic isocyanates attack the 5-position aromatic isocyanates, however, attack the N-oxide-oxygen of the heterocyclic ring. Acid chlorides react with 5 giving reactive N-acylammonium salts which can be isomerisized by bases. As with isocyanates differences of the individual acid chlorides can be observed: with