A method previously developed to study the thermochemistry of weakly bound molecular complexes in condensed phase has been used to investigate the gas phase molecular complex that forms between benzene and oxygen. Observed changes in absorbance in benzene-oxygen mixtures with temperature are attributed to changes in complex concentration. Based on this, Delta H-rxn, the enthalpy change for complex formation, is found to be -4.3 +/- 1.0 kJ/mol, indicating that the complex is a "contact charge transfer" complex. The result from the present study is compared to two previous determinations of Delta H-rxn for the benzene-oxygen complex and is used to resolve the discrepancies in the previous results.