[en] ESR investigations of γ-irradiated aqueous glasses (8 M NaClO₄, 12 M LiCl) containing DNA bases and analogues show that oxidizing species produced in the glasses (hydroxyl radicals and Cl₂^-) at 77 K attack a number of the bases and their analogues after warming to produce π-cation radicals. Compounds which produce π cations include thymine, uracil, 3-methyluracil, 6-methyluracil, orotic acid, isoorotic acid, guanine, and adenine. Compounds which are not found to produce π cations are 1-methylthymine, thymidine, and 1-methyluracil. The overall reaction for formation of the π cations is as follows: base + OH.(Cl₂^-) → OH^-(2Cl^-) + base cation. A pH and substituent dependence is also noted for the pyrimidines. At pHs where the nitrogen at position 1 is protonated, π-cation radicals are not formed. In addition, when there are substituents at position 1, π cations are not found. Analysis of the ESR spectra of the DNA base π cations for hyperfine splittings and g values were performed by aid of computer simulations. The analyses are in agreement with values found for several DNA base cations produced by photolysis and by direct γ irradiation