We have used double-laser nanosecond laser flash photolysis to study the kinetics of bimolecular quenching of the lowest doublet excited state of the transient diphenylmethyl radical (PhCH) in acetonitrile at 25 °C. We chose a series of cyanoaromatic compounds as acceptor-type quenchers and a series of 4-substituted phenols as donor-type quenchers. The observed bimolecular quenching rate constants ( k) are in the range 2 × 10-2 × 10 M s for cyanoaromatics and 1 × 10-3 × 10 M s for phenols. The Hammett plots of the k values for these two series have positive and negative slopes, respectively, and establish the enhanced behavior of the photo-excited diphenylmethyl radical as both a donor and an acceptor. For cyanoaromatics, the trend in the dependence of PhCH* quenching rate constants on the free energy change of electron transfer (Δ G) coincides well with that of Rehm-Weller data on acceptor-type and donor-type quenchers of singlet excited states. An approximate fit of the k data for cyanoaromatics to the Agmon-Levine model of electron transfer gave a value of 4.5 kcal mol for the free energy of activation at Δ G = 0. For phenols, the trend in the dependence of k on the free energy change of electron transfer deviates significantly from that of cyanoaromatics data. This deviation was traced to the use of irreversible oxidation potentials of phenols to calculate Δ G. No isotope effect is observed from substitution of phenolic hydrogen by deuterium, i.e., k/ k ≈ 1. Phenolate ions quench PhCH* with large rate constants, 1-2 × 10 M s, i.e., well in the limit of diffusion control. [ABSTRACT FROM AUTHOR]