characterized by the presence of a covalent 0-0 bond. Their highly complex chemistry has been extensively studied and can only be summarized here, with stress on those aspects which seem of particular interest in radiobiology. Comprehensive reviews are given by Tobolsky and Mesrobian (1) and by Davies (2). The writer has also discussed the radical chemistry of peroxides in some detail (3). Consideration here will be under three headings: peroxide synthesis, nonradical reactions of peroxides, and radical-forming decompositions, together with the typical reactions of the radicals produced. Because of the reviews available, only particularly significant or recent references will be cited.