[en] The relative roles of the radicals produced by water radiolysis in the inactivation of biotin in aqueous solution were investigated. The effects of nitrous oxide and isopropanol used as selective free radical scavengers allowed the inactivation efficiencies per unit G-value of OH, H, and esub(aq)^- to be estimated; these efficiencies were 0.73, 0.10, and 0.02 in neutral solution, respectively. Hydrogen gas and hydrogen peroxide unaffected the activity of biotin. G₀-Value for biotin inactivation in oxygen-free neutral solution was 2.08. Under these conditions the hydroxyl radical attack was found to be responsible for the large part of inactivation. On the other hand, in oxygenated neutral solution, G₀-value was 4.16. This large increase of inactivation in oxygenated solution suggested that, although hydrated electrons were considerably ineffective as an inactivating species in oxygen-free solution, superoxide ions would be much more effective in causing inactivation of biotin in oxygenated solution. A rate constant for the reaction of biotin with hydroxyl radical was 1.34 x 10¹⁰M^-1 sec^-1 as determined by the PNDA method. (auth.)