The bimolecular rate constants k(O2)(T) for oxygen (O-2((3) Sigma(g)(-))) quenching and the efficiencies f(Delta)(T) with which singlet oxygen (O-2(*)((1) Delta(g))) is thereby produced are reported for a range of substituted naphthalene triplet states in acetonitrile, benzene, and cyclohexane. The magnitudes of k(O2)(T) and f(Delta)(T) are inversely correlated, and both parameters exhibit pronounced sensitivity to the oxidation potential (E(M)(OX)) of the naphthalene derivative and some dependence an the solvent. Since, within the range of naphthalenes studied, the triplet state energy (ET) remains largely constant and the molecules are structurally similar, the dominant variable is the free energy change (Delta G(CT)) for charge transfer to molecular oxygen. It is demonstrated that the large variations observed in k(O2)(T) and f(Delta)(T) depend on the energy of the substituted naphthalene/molecular oxygen charge-transfer (CT) states, (1,3)(M(.+)...O-2(.-)). In acetonitrile, for example, the respective magnitudes of k(O2)(T) and f(Delta)(T) are 7.2 X 10(9) dm(3) mol(-1) s(-1) and 0.33 for 1-methoxynaphthalene compared with 1.4 X 10(9) dm(3) mol(-1) s(-1) and 0.74 for 1-cyanonaphthalene. In the nonpolar solvent cyclohexane, the CT state energy levels are raised (by similar to 14 kJ mol(-1)) relative to the energy levels in acetonitrile and benzene and this is reflected in decreased oxygen quenching rate constants ((1-3) x 10(9) dm(3) mol(-1) s(-1)) and increased efficiencies of singlet oxygen production (0.56-1.0), particularly for those naphthalenes which contain electron-donating substituents. In all three solvents the k(O2)(T) and f(Delta)(T) values for naphthalenes containing strong electron-withdrawing substituents (e.g. -CN, -NO2) remain largely constant. In order to account for the observed data, it is necessary to invoke a potential barrier (Delta G double dagger) to charge-transfer formation or the formation of exciplexes with significant CT character in the quenching step.