It was found that the formation of 2,5′- and 2, 6′-azobenzimidazoles, formed during the action of nitrosyl sulfuric acid on 2-amino-2-alkyl (phenyl, benzyl) benzimidazoles, occurs by azo combination of the extremely active salt of benzimidazole-2-diazonium obtained at first with the original amine. The azo compound is formed from 2-aminobenzimidazole substituted in position 5 and 6 only in the presence of 2-dimethylamino-1-methylbenzimidazole or other azo constituents. In the presence of substitutes, which hinder azo combination, 2,2′-diazoaminobenzimidazoles stable also in strongly acid medium may constitute the product of the reaction.