An ESR study of the relativedistribution of ion radicals formed in DNA equilibrated with D20 and-irradiated at 77 K is presented. The ESR spectra of irradiated DNA and polynucleotides (poly [dG]-poly [dC] and poly [dAdT]-poly] dAdT]) were obtained and employed in a computer-assisted analysis for the individual ion-radical distribution. Analysis of spectra as a function of power allowed the separation of the spectra of the pyrimidine anions (T*-, C*-) from thespectra of the purine cations (G,+, A*+). The spectra of the mononucleotide ion radicals, dCMP'", dTMP*", dGMP,+, and dAMP’+, were produced in 8 M LiCl glasses. In addition, thespectra of the ion radicals of all of the mononucleotide ion radicals except dAMP'+ were simulated by using hyperfme andg tensors from the literature. Basis spectra derived from (1) power saturation experiments,(2) polynucleotideand mononucleotide spectra,(3) spectra of mononucleotides alone, and (4) anisotropic simulations were used to fit the spectra of DNA by use of a linear least-squares analysis. Each of the four separate analyses confirmsthat the cytosine anion dominates the spectra of DNA at 100 K. Three analyses include thecationic composition, and they strongly favor the guanine cation over the adenine cation. An average of our results gives the DNA ion radicals’ relative abundances as ca. 77% C*~, 23% T*~ for theanions and> 90% G ‘+ for the cations; about equal amounts of anions and cations are present. No difference in results is found for DNA irradiated in frozen DzO solutions or simply exchanged at 100% D20 humidity.