The reactions of oxidizing radicals (OH)-O-center dot, N-3(center dot) Br center dot-, and NO2 center dot with tryptamine (Tpe) and 5-hydroxytryptamine (HTpe) were studied by pulse radiolysis and analyzed by quantum chemical calculations. Barring NO2 center dot radical, the rate constants for their reaction with Tpe and HTpe were found to be diffusion controlled and the rates in the NO2 center dot radical reaction with HTpe are lower by 2 orders of magnitude with k similar to 1 x 10(7) dm(3) mol(-1) s(-1). The transient spectra formed on oxidation of Tpe and HTpe exhibited peaks at 330 and 530 nm (indolyl radical) and 420 nm (indoloxyl radical), respectively, and the latter is in reasonable agreement with the calculated value (407 rim). Both radicals decay through direct recombination, but only the indoloxyl radical was observed to react with the parent molecule to give a (HTpe-Ind)(center dot) radical adduct for [HTpe] >= 50 x 10(-6) mol dm(-3). The calculated optimized geometries in water revealed the formation of two distinct types of radical adducts, one through the H-O bond and the other by C-C linkage. The H-O bonded radical adduct was found to be exothermic with a reaction enthalpy of -4 kcal mol(-1) and bond length 0.1819 nm and the C-C bonded radical adducts are endothermic and rate determining but are finally driven by exothermic processes involving intermolecular H transfer followed by intramolecular reorganization through H shift resulting in stable C4-C4' and C2-C4' dimers with reaction enthalpies of -39 and -44 kcal mol(-1), respectively, and this process was found to be thermodynamically as efficient as direct recombination of indoloxyl radicals. The formation of the two dimer products was also seen in steady-state radiolysis. The lack of adduct formation in the case of indolyl radical with Tpe is due to the positive free energy change (Delta G = 10 kcal mol(-1)). The energetics for the (OH)-O-center dot addition have shown dependence on the site of activation with (HTpe-OH)(center dot) adducts at C2 and C4 and the (Tpe-OH)(center dot) adduct at C2 being more thermodynamically stable and the water elimination to give the indoloxyl radical proceeds fast from (HTpe-OH)(center dot) adduct at C4 due to favorable geometry.