To characterize the relative reactivities of different dye tautomeric forms of model azo dyes, pulse radiolysis studies were made of their reactions with either oxidizing radicals (hydroxyl radical ((OH)-O-.) and N-3(.)) or reducing species (e(aq)(-) and the 2-hydroxy-2-propyl radical) in the pH range 3 - 12. Methyl orange, 2-(arylazo)-1-naphthol-3,6-disulfonate, and orange I or its O-methylated derivative undergo direct one-electron oxidation with N-3(.) and one-electron reduction with either eaq- or the 2-hydroxy-2-propyl radical. The efficiency of one-electron oxidation of the dyes is strongly dependent upon their different tautomeric forms in the order common ion > hydrazone > azo. In contrast, the ease of reduction of the different tautomeric forms of the dyes is in the order protonated azo greater than or equal to hydrazone > azo > common ion but shows a weaker dependence than that seen for oxidation. Radical intermediates formed either from one-electron oxidation or from one-electron reduction of the dyes mainly decay bimolecularly to give product(s) through radical-radical disproportionation. The reactivity of the (OH)-O-. is independent of the tautomeric forms of the dyes, forming (OH)-O-. adducts that decay bimolecularly to give addition product(s) by radical-radical combination (dimerization) at high radical concentrations. However, intermediates formed from orange I and its O-methylated derivative exhibit a competing hydroxyl ion elimination to give the one-electron-oxidized species, which decay by disproportionation, a less efficient process than bimolecular dimerization. Thus, the efficiency of removal of the dye by the (OH)-O-. is critically dependent on the pH, which governs the competition between the bimolecular dimerization of the (OH)-O-. adducts and the first-order water elimination pathway.