A proton-coupled, photoinduced electron transfer mechanism is responsible for the extraordinarily efficient dynamic and static quenching (94-99%) of the fluorescence of the pyrenyl residue (Py) in the benzo[a]pyrene metabolite 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT) by the 2’-deoxynucleosides dG, dC, and dT in aqueous solutions. Time-correlated fluorescence single-photon counting techniques indicate that noncovalent [(BPT)-B-1-dN] complexes decay with lifetimes 200-300 faster than those of free singlet excited (BPT)-B-1 molecules. An unusual solvent kinetic isotope effect is observed : these lifetimes are longer by factors of 1.5-2.0 in D2O than in H2O. Nanosecond time scale transient absorption techniques show that BPT.+ radical cations are formed with yields phi(R) = 0.07 and 0.02 in 0.1 M aqueous dC and dT solutions, respectively, with similar yields of (BPT)-B-3 triplet excited states. In the case of dG, the products of the quenching reaction are BPT.- radical anions (phi(R) = 0.25) and (BPT)-B-3 (phi(T) = 0.35) in dimethyl sulfoxide (DMSO); in aqueous solutions, however, only (BPT)-B-3 triplet excited states are observed on nanosecond time scales. This lack of ion radical products is accounted for in terms of a more rapid recombination of the intermediate [BPT.-...dG(.+)] radical-ion pair, which is facilitated by hydrophobic interactions in water, The striking difference in the directions of electron transfer from (BPT)-B-1 to the pyrimidines dC and dT on the one hand, and from the purine derivative dG to (BPT)-B-1 on the other, can be rationalized in terms of the redox potentials of the relevant donor-acceptor pairs. In the case of dC and dT, the thermodynamics of electron transfer are unfavorable unless coupled to a rapid proton transfer step; this effect accounts for the strong quenching in water and the kinetic solvent isotope effect, as well as for the observed lack of fluorescence quenching by dC and dT in the polar organic solvent DMSO.