Reduction reactions of 3,5-diiodotyrosine (DTR) are reported in phosphate-buffered aqueous medium. Steady state gamma-radiolysis and electron pulse irradiation have been used to study reduction mechanisms of DTR in the presence of primary radicals, e(aq)(-) and H-., and secondary radicals derived from oxygen, HO2. and O-2(-), in acidic, neutral, and alkaline media. The rate constant for H-. addition to the DTR phenolic chromophore is estimated to be k = 2.4 x 10(9) at pH 1.8 and 1.6 x 10(9) dm(3) mol(-1) s(-1) at pH 5; a cyclohexadienyl type transient with peak absorption at 385 nm and epsilon(385) = 900 +/- 200 dm(3) mol(-1) cm(-1) is produced in both cases. Reactions of e(aq)- on the other hand show a marked pH dependence. Electron addition to the phenolic chromophore between pH 4 and 12 produces a transient species that absorbs below 300 nm with lambda(max) at 280 nm and epsilon(280) = 9300 +/- 700 dm(3) mol(-1) cm(-1) with concurrent liberation Of I-. G(I-) is constant over the pH range and is estimated to be 0.11 +/- 0.01 mu M Gy(-1). In addition, in acidic and neutral media, ammonia elimination occurs with G(NH3) = 0.19 at pH 4 and 0.12 mu M Gy(-1) at pH 7. The rate constants for these processes are intermediate to the rate, 8.0 x 10(9) dm(3) mol(-1) s(-1) for e(aq)- addition to the -NH3+ group (subsequently leading to deamination) and 1.55 x 10(10) dm(3) mol(-1) s(-1) for addition to the phenolic chromophore. DTR does not react with HO2. and O-2(-) radicals. The reducing a-hydroxy radical derived from isopropyl alcohol, (H3C)(2)(COH)-O-., and carboxyl radical, CO2-, do not react with DTR by either electron transfer, H-abstraction, or addition to the phenolic ring, indicating a reduction potential value of the DTR(-)/DTR couple more negative than -2.0 V vs NHE.