A comparative study of some isocyanide dichlorides R‐NCCl2 and their isomeric N‐chloro‐formimidoyl chlorides R‐C(Cl) NCl is described. For this purpose we tried to synthesize N‐cyano‐isocyanide dichloride (III) and diisocyano tetrachloride (tetrachloroformalazine) (IX). Chlorination of S,S′‐dimethyl cyanodithiocarbonimidate (II) did not yield III but the dimeric 2,4‐dichloro‐6‐dichloromethylimino‐s‐triazine (IV) (tetramer of cyanogen chloride). IX was prepared by exhaustive chlorination of tetramethylthioformalazine (VI).