Both excited singlet states (1)Sigma (+)(g) and (1)Delta (g) and the unexcited triplet ground state (3)Sigma (-)(g) of molecular oxygen are formed with varying rate constants k(T)(1 Sigma), k(T)(1 Delta) and k(T)(3 Sigma) respectively, during the quenching by O-2 Of triplet states T-1 of sufficient energy ET The present paper reports these rate constants for a series of nine naphthalene sensitizers of very different oxidation potential, E-ox but almost constant ET These data complement data for k(T)(1 Sigma) k(T)(1 Delta), and k(T)(3 Sigma), determined previously for 13 sensitizers of very different ET. The analysis of the whole set of rate constants reveals that the quenching of triplet states by O-2 results in the formation of O-2((1)Sigma (+)(g)), O-2((1)Delta (g)), and O-2((3)Sigma (-)(g)) With varying efficiencies by two different channels, each capable of producing all three product states. One quenching channel originates from excited (1,3)(T(1)(.3)Sigma) complexes without charge-transfer character (nCT), which we cannot distinguish from encounter complexes; the other originates from (1)(T(1)(.3)Sigma) and (3)(T(1)(.3)Sigma) exciplexes with partial charge-transfer character (pCT). Rate constants of formation for O-2((1)Sigma (+)(g)), O-2((1)Delta (g)), and O-2((3)Sigma (-)(g)) are controlled by the respective excess energies via an energy gap relation in the nCT channel, whereas they vary with varying free energy of complete electron transfer in the pCT channel. A fast intersystem crossing equilibrium between (1)(T(1)(.3)Sigma) and (3)(T(1)(.3)Sigma) is surprisingly observed only in the nCT but not in the pCT channel.