Reaction of 5-diazo-4-nitroimidazole with concentrated hydrochloric acid leads to 4-chloro-5-diazoimidazole. The same product is formed together with a small quantity of 5-chloro-4-nitroimidazole with 1 N hydrochloric acid, but with 0.1 N hydrochloric acid only chloronitroimidazole was isolated. The results obtained are explained with the aid of quantum chemical calculations.