The rate constants, k(T)(O2), for oxygen quenching in acetonitrile of the triplet states of biphenyl and nine of its derivatives and the efficiencies of formation thereby of singlet oxygen, f(D)elta(T), have been measured as have the oxidation potentials of these derivatives. The rate constants k(T)(O2) decrease from 12.6 x 10(9) to 0.88 x 10(9) dm(3) mol(-1) s(-1) as the oxidation potentials of the biphenyls rise from 1.30 to 2.11 V vs SCE while f(Delta)(T) varies in the opposite direction rising from 0.31 to 0.84 with increasing oxidation potential. The mechanism of quenching via singlet and tripler complexes is discussed. The energy of the charge-transfer state, involving electron transfer to oxygen, relative to the energy of the locally excited triplet state, is established as important in determing k(T)(O2) and f(Delta)(T). The free energy of activation for charge-transfer-assisted quenching by oxygen via singlet and triplet channels is shown to have a linear dependence on the free energy change for full charge transfer, but the indications are that quenching is via singlet and tripler charge-transfer complexes with only partial (about 13.5%) charge-transfer character.