The enthalpy of reaction for the formation of the gas-phase complexes fluorobenzene-O-2 and toluene-O-2 has been determined using a spectroscopic method previously applied to the benzene + O-2 system. The results are Delta H-rxn = - 5.4 +/- 2.1 kJ/mol for the fluorobenzene-O-2 complex and Delta H-rxn = + 1.1 +/- 0.9 kJ/mol for the toluene-O-2 complex. Attempts to observe complexes of oxygen with chlorobenzene and the three xylene compounds were unsuccessful, most likely due to the low vapor pressure of the donor molecules and small value for the equilibrium constant for complex formation. Results are compared to those previously obtained for other aromatic-O-2 complexes.