UV and Visible absorption spectra of amino azobenzene and dimethyl amino azobenzene showed the similar behaviors under variations of PH, substituents, and oxidants. The maximal absorption band around 350 nm seen in both amino azobenzenes was found to relate closely with the quarterization by the proton addition or the N-hydroxylation by the oxidation, of the amine substituent. The fact that an ESR spectrum of azobenzene aminyl-1-oxy radical is observed in oxidized amino azobenzene suggests a possibility for N-hydroxylation of amino azobenzene under oxidation and to involve the same pathway as the metabolic activation of carcinogenic dimethyl amino azobenzene.