Rate constants of formation of O-2((1)Sigma(g)(+)), O-2((1)Delta(g)), and O-2((3)Sigma(g)(-)) in the quenching of triplet states T-1 by O-2 have been determined for a series of nine benzophenones (BPs) of strongly varying oxidation potential, E-ox, but almost constant triplet-state energy E-T. These data are analyzed considering data determined previously for T-1(pipi*) sensitizers of very different E-T and E-ox. Much weaker charge transfer (CT) effects are observed for the T-1(npi*) BPs compared with those obtained with a series of structurally related T-1(pipi*) biphenyls. The quenching proceeds for T-1(npi*) and T-1(pipi*) sensitizers via two different channels, each capable of producing O-2((1)Sigma(g)(+)), O-2((1)Delta(g)), and O-2((3)Sigma(g)(-)). One channel originates from excited (1,3)(T(1)(.3)Sigma) complexes with no CT character and the other from (1,3)(T(1)(.3)Sigma) exciplexes with partial CT character. Different energy gap relations determine the formation of O-2((1)Sigma(g)(+)), O-2((1)Delta(g)), and O-2((3)Sigma(g)(-)) of T-1(pipi*) and T-1(npi*) sensitizers in the nCT channel, whereby the excess energy (DeltaE) dependence of the corresponding rate constants is much weaker for the T-1(npi*) ketones. In the pCT channel, the respective rate constants vary on a logarithmic scale linearly with the free energy change for complete electron transfer for both T-1(pipi*) and T-1(npi*) sensitizers. This dependence too is much weaker for T-1(npi*) than for T-1(pipi*) sensitizers. The comparison with CT induced quenching of O-2((1)Deltag) by ground-state sensitizers reveals that the different electronic configurations leads to different sterical structures of (1,3)(T-1(npi*)(.3)Sigma) and (1,3)(T-1(pipi*)(.3)Sigma) complexes. These differences strongly influence the complex deactivation and explain both the weaker DeltaE dependence and the weaker CT effects in the quenching of T-1(npi*) by O-2.