[en] The γ-radiolysis of de-aerated neutral aqueous solutions of uracil, thymine, cytosine and of the corresponding nucleosides and nucleotides and of calf-thymus DNA was investigated. For uracil and thymine, the U.V. photolysis of aqueous solutions containing H₂O₂ was also studied. The short-lived radicals were spin-trapped by tert-nitrosobutane and identified by electron-spin-resonance spectroscopy. For all compounds two or more radicals were observed, and these could be distinguished by following the thermal decay of the spin adducts. Radicals formed by the addition of H or OH at the C(5) or C(6) positions of the pyrimidine derivatives were observed in all classes. Sodium formate was used as a scavenger for H and OH to identify the radicals formed by e^-sub(aq). Spin-trapped radicals in γ-irradiated aqueous solutions of polynucleotides exhibited broad e.s.r. lines. For DNA gel, additional narrow lines due to scission products were also found. (author)