The kinetics of triplet state quenching of 3,3',4,4'-benzophenone tetracarboxylic acid (BPTC) by DNA bases and thymidine has been investigated adenine, adenosine, thymine, in aqueous solution using time-resolved laser flash photolysis. The observation of the BPTC ketyl radical anion at lambda(max) = 630 nm indicates that one electron transfer is involved in the quenching reactions. The pH-dependence of the quenching rate constants is measured in detail. As a result, the chemical reactivity of the reactants is assigned. The bimolecular rate constants of the quenching reactions between triplet BPTC and adenine, adenosine, thymine, and thymidine are k(q) = 2.3 x 10(9) (4.7 < pH < 9.9), k(q) = 4.0 x 10(9) (3.5 < pH < 4.7), k(q) = 1.0 x 10(9) (4.7 < pH < 9.9), and k(q) = 4.0 x 10(8) M-1 s(-1) (4.7 < pH < 9.8), respectively. Moreover, it reveals that in strong basic medium (pH = 12.0) a keto-enol tautomerism of thymine inhibits its reaction with triplet BPTC. Such a behavior is not possible for thymidine because of its deoxyribose group. In addition, the pH-dependence of the apparent electrochemical standard potential of thymine in aqueous solution was investigated by cyclic voltammetry. The Delta E/Delta pH approximate to -59 mV/pH result is characteristic of proton-coupled electron transfer. This behavior, together with the kinetic analysis, leads to the conclusion that the quenching reactions between triplet BPTC and thymine involve one proton-coupled electron transfer.