Employing electron spin resonance (ESR) spectroscopy, we have characterized the radicals formed in 3'-azido-3'-deoxythymidine (3'-AZT) and in its 5'-analog 5'-azido-5'-deoxythymidine (5'-AZT) after electron attachment in gamma-irradiated aqueous (H2O or D2O) glassy (7.5 M LiCl) systems. ESR spectral studies and theoretical calculations show that the predominant site of electron capture in 3'-AZT and in 5'-AZT is at the azide group and not at the thymine moiety. The azide group in AZT is therefore more electron affinic than the most electron affinic DNA base, thymine. Electron attachment to 3'-AZT and 5'-AZT results in an unstable azide anion radical intermediate (RN3 center dot(-)) that is too short-lived to be observed in our work even at 77 K. At 77 K, we observe the neutral aminyl radical (RNH center dot) after loss of N-2 from RN3 center dot(-) followed by protonation of nitrene anion radical (RN center dot(-)) to give RNH center dot. The expected RN center dot(-) intermediate is not observed as protonation from water is complete at 77 K even under highly basic conditions. Formation of RND center dot in D2O solutions confirms water as the source of the NH proton in the RNH center dot. Our assignments to these radicals are aided by DFT calculations for hyperfine coupling constants that closely match the experimental values. On annealing to higher temperatures (ca. 160-170 K), RNH. undergoes bimolecular hydrogen abstraction reactions from the thymine methyl group and the sugar moiety resulting in the formation of the thymine allyl radical (UCH2 center dot) and two sugar radicals, C3'center dot and C5'center dot. RNH center dot also results in one-electron oxidation of the guanine base in 3'-AZG. This work provides a potential mechanism for the reported radiosensitization effects of AZT.