The kinetics of the C6H5 reaction with CO has been studied by the cavity ring-down spectrometric technique in the temperature range 295-500 K at 12-120 Torr pressure with Ar as the carrier gas. The reaction occurred near the high-pressure limit under the conditions used. A weighted least-squares analysis of all data gives the rate constant for the association reaction, k1 = 1011.93±0.14 exp[-(1507 ±109)77] cm3 mole-1 s-1), where the errors represent two standard deviations. Our result can correlated satisfactorily with the kinetic data reported by Solly and Benson for the reverse process (J. Am. Chem. Soc. 1971, 93, 2171) with the Rice-Ramsperger-Kassel-Marcus (RRKM) theory using the transition-state parameters computed quantum mechanically by the MP2 method with the 6-31G(d,p) basis set. Combination of the forward and reverse reaction data gives ΔH°1 = -24.6 ± 0.8 kcal/mol at 0 K and k-1∞, = 5.3 × 1014 exp(-14 600/T) s-1 for the temperature range 300-670 K. The heat of reaction, combining with the known heats of formation of the reactants, leads to ΔfH°0 (C6H5CO) = 32.5 ± 1.5 kcal/mol.