A large set of literature kinetic data on triplet (T-1) sensitization of singlet oxygen by two series of biphenyl and naphthalene sensitizers in solvents of strongly different polarity has been analyzed. The rate constants and the efficiencies of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a noncharge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of 1( T1 3 T) and 3( T1 3 T) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O-2((1)Sigma(+)(g)), O-2((1)Delta(g)), and O-2((3)Sigma(-)(g)). IC of (1,3)(T-1(3)Sigma) encounter complexes is controlled by an energy gap law that is generally valid for the transfer of electronic energy to and from O2. 1,3( T1 3 T) nCT complexes form in competition to IC 1( T1 3 T) and 3( T1 3 T) exciplexes if CT interactions between T1 and O2 are important. The rate constants of exciplex formation depend via a Marcus type parabolic model on the corresponding free energy change Delta G(CT), which varies with sensitizer triplet energy, oxidation potential, and solvent polarity. O-2((1)Sigma(+)(g)), O-2((1)Delta(g)), and O-2((3)Sigma(-)(g)) are formed in the product ratio (1/6):(1/12):(3/4) in the CT deactivation channel. The balance between nCT and CT deactivation is described by the relative contribution pCT of CT induced deactivation calculated for a sensitizer of known triplet energy from its quenching rate constant. It is shown how the change of pCT influences the quenching rate constant and the efficiency of singlet oxygen formation in both series of sensitizers. pCT is sensitive to differences of solvent polarity and varies for the biphenyls and the naphthalenes as sigmoidal with Delta G(CT). This quantitative model represents a realistic and general mechanism for the quenching of pi pi triplet states by O-2, surpassing previous advanced models.