Reactions of H, OH, and e(aq)(-) have been studied with 2,5-dimercaptothiadiazole (DMTD) in aqueous solutions by the pulse radiolysis technique. The reaction with all the primary species as well as some one-electron oxidants and reductants produced the same species, viz., a resonance stabilized radical with electron delocalized over the -N-C-S bond. The reaction with one-electron oxidizing radicals is by electron transfer. In the case of hydrated electrons and H atoms, there is initially elimination of H2S giving a mercaptothiadiazolyl radical, which subsequently reacts with the parent molecule by H abstraction from the -SH group to give a radical identical to that formed by reaction with one-electron oxidant species such as OH, N-3 . radicals, etc. From the equilibrium with I-2(.-) radicals the one-electron reduction potential of the oxidized species was determined to be 0.8V vs NHE. COOH and (CH3)(2)COH radicals react with DMTD at acidic pH’s by H abstraction to give the resonance-stabilized neutral thiyl radical as obtained by OH radical reaction at pH 0.