In connection with the carcinogenic activity of 4- (hydroxyamino) quinoline N-oxide (4HAQO), voltammetric studies of the oxidation processes of 4HAQO and related substances were carried out in aqueous solutions. It was verified that 4HAQO forms a reversible redox couple with 4-nitrosoquinoline N-oxide (4NOQO) through an intermediate free radical. The reversible reactions in these redox couples were evaluated quantitatively, and the formation constant of the intermediate free radical was calculated. The pH dependence of the above standard redox potential gave the pKa values, which agreed quite well with the data measured by ultraviolet spectral methods. The molecular structures of species formed by proton addition or dissociation were investigated by analyses of the pKa values and the electronic spectra, and by their molecular orbital calculations. The N-oxide type structure was considered to be a main species in aqueous solutions of 4HAQO and its analogues.