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Purpose: To study the direct effect of heavy ions and electrons on thymidine., Materials and Methods: The thymidine samples in the solid state were exposed to a beam of O7+ heavy ions with an energy of 10.6 MeV/u (LET approximately 500keV/microm) and to electrons of 2MeV (LET approximately 0.18keV/microm). The major decomposition products of thymidine were purified by high performance liquid chromatography (HPLC) and identified by extensive spectrometric measurements (UV, mass spectroscopy, 1H and 13C NMR)., Results: The main degradation products of thymidine were isolated and characterized. Reaction mechanisms, involving transient radical species, are proposed to explain the heavy ion-mediated formation of the modified products. Furthermore, a semi-quantitative comparison of the modifications induced within thymidine by the two types of radiations was performed., Conclusion: Several new radiation-induced thymidine decomposition products have been isolated and characterized. The comparison of the effects induced by heavy ions and electrons on thymidine in the solid state clearly indicates several significant differences in the mechanisms of action. A relative increase in the extent of the modifications of the sugar moiety with respect to those of the base is observed with the heavy ions by comparison with electrons.