Biothiols are extraordinarily efficient antioxidants protecting the cells against consequences of damage induced by free radicals, due to their ability to react with the latter. In such antioxidant reactions, thiols undergo one-electron oxidation with the formation of thiyl radicals. For this reason, attention has been focused mostly on antioxidant properties of thiols. Considerably less attention has been paid to thiyl radicals (RS*) formed simultaneously in these reactions. However, protective and repairing efficacy of thiols depends not only on their capacity to detoxify free radicals but also on chemical character and reactivity of the formed thiyl radical. Furthermore, quick and efficient removal of RS radical leads to a disturbance in balanced state of antioxidant reaction, which effectively increases repairing capacity. Dangerous thiyl radicals, which can cause peroxidative injury, should immediately undergo regenerative reduction to thiols. Under physiological conditions, thiyl radicals can react with thiolate anion yielding disulfide radical anion (RSSR)-* as an intermediate and finally disulfides and superoxide radical anion (O2-*), which is next inactivated in the reaction catalyzed by superoxide dismutase (SOD). Thiyl radicals can also be reduced to thiols by reacting with ascorbate with the formation of low-activity ascorbyl radical, that subsequently enters disproportiation reaction.