The photochemical excitation of aqueous solutions of Ru(bpy)(3)(2+) and phenol (PhOH) in the presence of air produces 1,4-benzoquinone (BQ) as the only organic product. In this study, we examined the *Ru(bpy)(3)(2+)/PhOH/O-2 system in terms of the dependence of the quantum yield of BQ formation (Phi(BQ)) as a function of [O-2], [PhOH], temperature, pH, and the composition of the solvent. The increase in Phi(BQ) from its low value in acidic solution to its maximum at pH similar to 9.4 is attributed to the increasingly competitive quenching of *Ru(bpy)(3)(2+) by PhO- and O-2. A maximum in Phi(BQ) is also observed at similar to 45 degrees C, an effect caused by the variation in the solubility of O-2 in solution as well as the activation energies of the competing steps in the mechanism. The proposed mechanism of the reaction, which could involve the formation of an endoperoxide intermediate from the reaction of O-1(2)% by PhOH, is consistent with the observed decrease of Phi(BQ) as a function of the mole fraction of H2O in CH3CN-H2O solvent mixtures.