The reaction Hg(63P)+O2 was investigated by modulation kinetic spectroscopy, in excess inert gas and in the presence of small partial pressures of glyoxal, biacetyl, and naphthalene. These energy acceptors were excited efficiently to their lowest triplet states by electronic energy transfer from a long-lived (τ ≥ 5 μsec) excited intermediate formed in the Hg*-O2 reaction. Phase shift measurements showed the rate of energy transfer to be quite fast, as indicated by the rate constants (at 1 atm and 313°K, in 1011 liters/mole sec) 0.96±0.19, 0.63±0.13, and 2.1±0.5 for glyoxal, biacetyl, and naphthalene, respectively. Overall efficiencies for transfer from Hg* via the intermediate to the acceptors were 0.4, 0.2, and 0.2 in the same order. Rate constants for quenching of the acceptor triplet states by O2 were also determined. The intermediate appears to react with O2 by a process whose rate varies with the square of the O2 pressure. It is not identical with vibrationally excited ground state O2 whose absorption spectrum is also detected during the reaction. This is shown by the effect of CO2 which quenches the vibrational excitation while leaving the energy transfer unaffected. It is concluded that the intermediate is an O2 molecule in one of the A, C, or c states or, less likely, an excited HgO2 complex. © 1978 American Institute of Physics.