The quenching of triplet C-60 and tetracene fluorescence by phenols is strongly enhanced by added pyridines. Evidence that this is due to quenching by hydrogen-bonded phenol-base pairs is given by the close agreement between equilibrium constants for hydrogen-bond formation derived from kinetic measurements and from independent spectroscopic data. The effect is attributed to a trimolecular transition state in which electron transfer from the phenol to the excited molecule is concerted with proton movement from the incipient strongly acidic phenol cation radical to the hydrogen-bonded base.