The sulfite radical anion, SO₃•‾, was easily generated by chemical (Ce(IV)-HSO₃‾ in acidic medium or Ti(III) (EDTA)-H₂O₂-SO₃ in alkaline medium) or biochemical (horseradish perexidase-H₂O₂) methods. The SO₃ radical was identified by electron spin resonance (ESR) spectroscopy coupled with a rapid-mixing flow technique as a direct detection or a spin-trapping method as an indirect detection. The life-time of the SO₃‾ radical is about 100 msec in aqueous solutions. The SO₃ radical could not abstract a hydrogen atom from the saturated compounds, but easily add to the unsaturated compounds having C=C, C=S, C≡C and C=N bonds. In the case of addition to C=S bond, it is suggested that the SO₃ radical is the active species for the chemical modification of biologically active nucleic acids by the bisulfite (HSO₃‾). Further, the biological reactivities of the SO₃ radical have been briefly summarized.