The charge-transfer induced quenching processes of the lowest excited triplet state (T-1) of naphthalene derivatives by ground-state oxygen and of singlet oxygen O-2((1)Delta (g),) by ground state naphthalene derivatives have been investigated in three solvents of different polarity. Both deactivation processes are described by one common Marcus type plot. The analysis of the data strongly indicates that exciplexes with the same degree of average partial charge transfer (pCT) and the same reorganization energy are formed in the rate-determining step of both quenching processes. The free energies of pCT complex formation are related to the corresponding free energy of complete electron transfer by DeltaG(CT) = f DeltaG(CET) With a common corrective factor f, for T-1 and O-2((1)Delta (g)) deactivation. The reorganization energy increases from 34 kJ mol(-1) in carbon tetrachloride to 92 kJ mol(-1) in acetonitrile. The charge transfer character is shown to be significantly larger than 25% and to increase with increasing solvent polarity.