Abstract: This invention relates to a novel method of oxidizing hydrazo compounds to corresponding azo compounds, and more particularly 4,4'-hydrazo-bis-4-cyano-pentanoic acid to 4,4'-azo-bis-4-cyano-pentanoic acid at a temperature close to ambient temperature, said method comprising the step of effecting oxidation with the aid of hydrogen peroxide under the following conditions: catalysis of the reaction by means of UV radiation, operation in aqueous solution at a pH.ltoreq.0.5, said pH being obtained by addition of hydrochloric acid and/or hydrobromic acid, and operation in a reaction medium comprising Br.sup.- ions at a concentration at least equal to 30 ppm.