The process for the preparation of 2(3H)-benzothiazolones which are optionally substituted on the benzene ring by reacting a 2-aminobenzothiazole which is optionally substituted on the benzene ring with a diazotization agent in the presence of aqueous hydrochloric acid, if appropriate with addition of a chloride, to give a diazonium chloride, reacting the diazonium chloride directly to give a 2-chlorobenzothiazole, which is optionally substituted on the benzene ring, and hydrolyzing the 2-chlorobenzothiazole, without intermediate isolation, at 120.degree.-200.degree. C. under reaction pressure.