Neutral sugar radicals formed in DNA sugar-phosphate backbone are well-established as precursors of biologically important damage such as DNA strand scission and cross-linking. In this work, we present electron spin resonance (ESR) evidence showing that the sugar radical at C5' (C5'(•)) is one of the most abundant (ca. 30%) sugar radicals formed by γ- and Ar ion-beam irradiated hydrated DNA samples. Taking dimethyl phosphate as a model of sugar-phosphate backbone, ESR and theoretical (DFT) studies of γ-irradiated dimethyl phosphate were carried out. CH(3)OP(O(2)(-))OCH(2)(•) is formed via deprotonation from the methyl group of directly ionized dimethyl phosphate at 77 K. The formation of CH(3)OP(O(2)(-))OCH(2)(•) is independent of dimethyl phosphate concentration (neat or in aqueous solution) or pH. ESR spectra of C5'(•) found in DNA and of CH(3)OP(O(2)(-))OCH(2)(•) do not show an observable β-phosphorus hyperfine coupling (HFC). Furthermore, C5'(•) found in DNA does not show a significant C4'-H β-proton HFC. Applying the DFT/B3LYP/6-31G(d) method, a study of conformational dependence of the phosphorus HFC in CH(3)OP(O(2)(-))OCH(2)(•) shows that in its minimum energy conformation, CH(3)OP(O(2)(-))OCH(2)(•), has a negligible β-phosphorus HFC. On the basis of these results, the formation of radiation-induced C5'(•) is proposed to occur via a very rapid deprotonation from the directly ionized sugar-phosphate backbone, and the rate of this deprotonation must be faster than that of energetically downhill transfer of the unpaired spin (hole) from ionized sugar-phosphate backbone to the DNA bases. Moreover, C5'(•) in irradiated DNA is found to be in a conformation that does not exhibit β-proton or β-phosphorus HFCs.