Using the technique of pulse radiolysis, oxidation studies of 3,5-diiodotyrosine (DTR) with radicals generated in phosphate-buffered aqueous medium are reported. Secondary oxidizing radicals S0(4).(-), Br-., N-3., Tl(OH)(+), (DMS)(2)(+), CCl3O2., (CNS)(2).(-), Cl-2.(-), and Tl2+ oxidize DTR in neutral, mildly alkaline, and acidic solutions by an electron-transfer process to generate a phenoxyl-type radical transient with peak absorptions observed at 280 and 350 nm and epsilon(350) = 4400 +/- 300 dm(3) mol(-1) cm(-1). In acidic pH, another transient peak at 310 nm with epsilon(310) = 3500 +/- 150 dm(3) mol(-1) cm(-1) and a pK(a) value of 6.5 is observed due to the simultaneous formation of a radical cation intermediate. The formation rate constant values of these two transients lie between 5 X 10(8) and 2 x 10(10) dm(3) mol(-1) s(-1) for these secondary oxidizing radicals as well as for the primary .OH/.O- radicals. The .OH radical additionally generates a secondary transient in deoxygenated solutions (k(f) approximate to 1.7 x 10(7) dm(3) mol(-1) s(-1) at pH = 8) with its peak absorption centered at 650 nm, which shows two pK(a) values of 3.9 and 7. It has been assigned the dimer-type structure arising from the DTR reaction with its hydroxyl radical adduct transient. From the equilibrium studies with dehydrocysteine (thiyl) as the oxidizing radical, the reduction potential value for the couple DTR(+)/DTR has been measured to be +0.78 +/- 0.04 V vs NHE.