Room-temperature free radicals produced in four gamma-irradiated polycrystalline pyrimidine nucleosides (cytidine, deoxycytidine, uridine and deoxyuridine) were studied by an improved spin trapping and ESR methods with NtB (nitroso-t-butane) serving as the spin trap. The presence of three types of radicals was ascertained in all samples, i.e. —C(5)H—C(6) H_2—and —C(5)H_2—C(6)H— base radicals, and —C(5’)H_2— ribose radical evolving from HOC(5’)H—C(4’)H<. In addition, radicals in which the unpaired electron was located on ribose C(1’) and C(4’) sites were tentatively distinguished. Stress was laid on the discussion of the mechanism of formation of ribose radicals. Room-temperature free radicals produced in four gamma-irradiated polycrystalline pyrimidine nucleosides (cytidine, deoxycytidine, uridine and deoxyuridine) were studied by an improved spin trapping and ESR methods with NtB (nitroso-t-butane) serving as the spin trap. The presence of three types of radicals were ascertained in all samples, ie -C (5) H-C (6) H_2-and -C (5 ’) H-C (4’) H <. In addition, radicals in which the unpaired electron was located on ribose C (1 ’) and C (4’) sites were tentatively distinguished. Stress was laid on the discussion of the mechanism of formation of ribose radicals.