AbstractThe action of phosphorus oxychloride and sulphuryl chloride on 4‐chloro‐ and 4‐hydroxypyridine‐N‐oxide and on their 3‐chloro‐derivatives has been investigated.Under the conditions chosen the acid chlorides reacted differently. Phosphorus oxychloride replaced the hydroxyl group by chlorine and promoted the deoxidation of the N‐oxide group. Sulphuryl chloride removed the oxygen atom of the N‐oxide group, introducing at the same time chlorine at one of the α‐positions; further it replaced the hydrogen atoms at the β‐positions by chlorine, but not the hydroxyl group.A substance whose analysis pointed to the formula C5Cl7NO was isolated from the reaction mixture obtained by heating 4‐hydroxypyridine‐N‐oxide, its 3‐chloro, 3,5‐dichloro or 3‐nitro derivative with an excess of sulphuryl chloride. It is assumed to be 1,2,2′,3,3′,5,6‐heptachloro‐1,2,3,4‐tetrahydropyridone‐4,inter aliabecause of its reduction yielding 2,3,5,6‐tetrachloro‐4‐hydroxypyridine. The last‐mentioned compound was identified with preparations synthesized from pentachloropyridine and 4‐amino‐2,3,5,6‐tetrachloropyridine. The substance C5Cl7NO was also obtain