The kinetics of the C6H5+O2 reaction has been studied with the cavity-ring-down (CRD) method by monitoring the rate of C6H5O2 radical formation at 496.4 nm. The second-order rate constants measured under the conditions, 20 Torr ≤ P ≤80 Torr (Ar), 297 K ≤ T≤473 K, were found to be pressure-independent, with a small negative activation of 0.32 kcal/mol. A least-squares analysis of a dozen sets of data obtained by two different kinetic evaluation methods gives rise to ko2= 10-11.00±0.08exp[(+161 ± 66)/T] cm3/molecules where the errors represent one-standard deviations, evaluated with the weighting factors wi = (ki/σi)2. Under the conditions employed in the present work, the C6H5 + O2 reaction takes place primarily by the addition-stabilization process C6H5 + O2 ⟷ C6H5O2 (+M) ΒC6H5O. A search carried out at 575.4 nm for the production of C6H5O, which could be conveniently and sensitively detected by the CRD method, failed to detect the radical. This finding is fully consistent with the apparent large activation energy for the formation of C6H5O from the C6H5 + O2 reaction (6 kcal/mol) recently reported by Frank and co-workers (ref 30) using the atomic resonance absorption-shock tube method at T < 900 K.