The reaction of 2-phenyl- and 1-methyl-2-phenylindole with nitrogen dioxide or with nitrousacid (NaNO2-CH3COOH) in benzene leads mainly to the formation of the isonitroso and 3-nitroso indole derivatives, respectively. When reacted with nitrousacid, 1-methyl-2-phenylindole gives also the corresponding azo-bis-indole in good yields. The reaction of indole with nitrogen dioxide leads to 2-(indol-3-yl)-3H-indol-3-one