Rate constants for quenching by molecular oxygen of excited singlet and triplet states, k(S)(O2) and k(T)(O2), respectively, are reported for 12 aromatic hydrocarbons in acetonitrile. Measured values of k(S)(O2), except in the case of fluoranthene for which k(S)(O2) = 6.6 x 10(9) dm(3) mol(-1) s(-1), are in the range (2.3-4.3) x 10(10) dm(3) mol(-1) s(-1), i.e., close to 4.5 x 10(10) dm(3) mol(-1) s(-1), the average value obtained for k(d), the rate constant for diffusions controlled reactions of oxygen with aromatic hydrocarbons in acetonitrile. Values of k(T)(O2) vary from 0.24 to 5.6 x 10(9) dm(3) mol(-1) s(-1) Thus, k(T)(O2)/k(d) was found to be less than one-ninth for 11 compounds. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, f(Delta)(S) and f(Delta)(T), respectively, were also measured, as were the oxidation potentials of the hydrocarbons in acetonitrile. Values offs, were shown to be zero within experimental error for eight compounds and in the range of 0.27 +/- 0.05 for the ether four compounds. Three different methods, which gave good agreement, were used to measure values of f(Delta)(T) which were found to vary from 0.41 in the case of acenaphthene to 0.85 for anthracene. The fraction of excited singlet states quenched by oxygen which result in triplet states f(T)(O2) was also measured for all compounds and found to vary from 0.49 to 1.0. Combination of the total quenching rate constants with the fractional efficiencies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways in acetonitrile. The reasons for variations in these net quenching rate constants and thus in the fractional efficiencies for quenching by the various quenching pathways are discussed. Quenching of both excited singlet and triplet states by energy transfer and by charge-transfer assisted pathways are established. The logarithm of the net rate constants for quenching of the triplet states without energy transfer to oxygen for ii of the aromatic hydrocarbons shows a linear dependence on the free energy for full charge transfer from the triplet state, with a slope which indicates that the transition states for this quenching pathway only have about 13.5% charge-transfer character.