DNA catalyzes the reactions of many small molecules and assists enzymes catalyzing modifications of DNA itself. Previous work by the authors showed that hydronium ions constitute an important component of the counterion atmosphere surrounding polyelectrolytes in general and DNA in particular. It was proposed that local regions at the surface of DNA, termed acidic domains, might be responsible for the protonation of epoxides to form reactive intermediates. This acid catalysis is DNA-mediated in the sense that DNA associatively binds the reactant hydronium ions. DNA catalysis of the hydrolysis of the syn- and anti-7, 8-diol 9, 10-epoxides of the procarcinogen benzo[a]pyrene (syn- and anti-BPDE) has been investigated for over a decade. Jerina and co-workers have shown that, in the absence of DNA, the observed rate of hydrolysis (k(obs)) can be represented as the sum of a spontaneous, pH-independent contribution (k(0)) and an acid-catalyzed component (k(H)) : k(obs) = k(0) + k(H)[H+], where [H+] denotes the (bulk) hydronium ion concentration. : k(obs) = k(0) + k(H)[H+], where [H+] denotes the (bulk) hydronium ion concentration. : k(obs) = k(0) + k(H)[H+], where [H+] denotes the (bulk) hydronium ion concentration.