Rate constants k(q) of fluorescence quenching by molecular oxygen O-3(2)((3) Sigma(g)(-)) and the efficiencies of enhanced intersystem crossing Phi(T) and free-radical generation Phi(R) are measured for several aromatic hydrocarbons in acetonitrile. k(q) is the diffusion-controlled limit, k(diff) (=(3.2-3.7) x 10(10) M(-1) s(-1)), when the free energy change Delta G of actual electron transfer (ET) from the first excited singlet fluorescer (1)M(*) to O-3(2) is more negative than -0.8 eV. In the region where Delta G > -0.8 eV, k(q) is slightly smaller than k(diff). Phi(T) decreases from 1.0 to 0.4 with decreasing Delta G, whereas Phi(R) is null except for 2,6-dimethoxynaphthalene (DMN). In the case of DMN, the actual ET is highly exothermic (Delta G = -1.45 eV). Nevertheless, Phi(R) is as low as 0.003, while Phi(T) is as high as 0.43. As the lowest triplet energy of 2.70 eV for DMN is greater than the energy of 2.07 eV for the radical ion pair ((2)DMN(.+) + O-2(2).-), the triplet DMN cannot be produced by recombination of the radical ion pair. These facts indicate that fluorescence quenching by O-3(2) is not induced by long-distance ET but rather by exciplex formation. Since the molecular size of O-3(2) is considerably smaller than that of the aromatic molecule, the solvent reorganization energy of long-distance ET from an aromatic molecule to O-3(2) is considered to be extremely high. For this reason, long-distance ET cannot compete with exciplex formation in fluorescence quenching by O-3(2).