The chlorine dioxide (ClO₂) which is a free radical was easily generated from many reaction systems (Ti³⁺-NaCIO₃, Ti³⁺-NaCIO₂, or H₂SO₄-NaCIO₂) in aqueous solutions. Tne formation of the ClO₂ radical was detected by means of the rapid-mixing flow technique coupled with electron spin resonance (ESR) measurements. The ClO₂ radical thus formed could oxidize some aromatic amines to the corresponding cation radicals, whereas it could neither abstract the hydrogen atom from saturated compounds such as methanol, ethanol and isopranol nor add to unsaturated compounds having a double and triple bond. The ClO₂, radical can oxidize some aromatic amines to give the corresponding cation radicals. Further, the ClO₂ radical can oxidize ascorbic acid to yield the short-lived intermediate, ascorbic acid free radical. This result indicates that the ascorbic acid is a suitable reagent for detoxification of the ClO₂, radical, which is remaining in the drinking water, in the living body.