Rate constants k(q)(T) ranging between 0.3 X 10(9) and 15 x 10(9) M(-1) s(-1) for quenching of the lowest excited triplet state of several substituted aromatic hydrocarbons and ketones by molecular oxygen were measured in toluene. The efficiencies of singlet oxygen (O-2((1) Delta(g))) formation S-Delta were determined, ranging from 0.24 to 1.00. The results can be explained by CT interaction in that k(q)(T) and S-Delta correlate with the free enthalpy of electron transfer Delta G(el), calculated from the redox potentials of the sensitizer and O-2. Quenching probabilities for the singlet and triplet channel were determined allowing the calculation of(k(i)/k(-d)) (where i = en, energy transfer, or ic, internal conversion), which exhibit an approximately linear dependence on Delta G(el). In one case (9-methylcarbazole), direct evidence for encounter complex intersystem crossing is found and discussed in terms of reversible charge transfer (CT) complex formation and CT complex intersystem crossing to obtain a probability of the intersystem crossing process of 0.27.