ALTHOUGH the technique of electron spin resonance (ESR) has been useful in the identification of some of the secondary radical species formed in γ-irradiated DNA and proteins, difficulties have been experienced in the interpretation of the spectra of the primary radiolytic products. Two of these secondary species are the thymine radical (with an additional H-atom on C-6 of the thymine ring) in DNA (refs. 1–3) and the radical R—CH2—S in cystine-containing proteins4–6. Both have been postulated to be the result of electron reactions7–10. We have irradiated two model compounds, thymidine and cystamine, in frozen glasses of either ethanediol–water (1:1) or 6 M NaOH and, by comparing the ESR and optical absorption spectra, we have identified the anions produced.