Rate coefficients, k, for the gas-phase reaction CH3 CO + Cl2 → products (2) were measured between 253 and 384 K at 55–200 Torr (He). Rate coefficients were measured under pseudo-first-order conditions in CH3 CO with CH3 CO produced by the 248-nm pulsedlaser photolysis of acetone, CH3 C(O)CH3 , or 2,3-butadione, CH3 C(O)C(O)CH3 . The loss of CH3 CO was monitored by cavity ring-down spectroscopy (CRDS) at 532 nm. Rate coefficients were determined by first-order kinetic analysis of the CH3 CO temporal profiles for [Cl2 ] < 1 × 1014 molecule cm−3 and the analysis of the CRDS profiles by the simultaneous kinetics and ring-down method for experiments performed with [Cl2 ] > 1 × 1014 molecule cm−3 . k2 (T ) was found to be independent of pressure, with k2 (296 K) = (3.0 ± 0.5) × 10−11 cm3 molecule−1 s−1 . k2 (T ) showed a weak negative temperature dependence that is well reproduced by the Arrhenius expression k2 (T ) = (2.2 ± 0.8) × 10−11 exp[(85 ± 120)/T ] cm3 molecule−1 s−1 . The quoted uncertainties in k2 (T ) are at the 2σ level (95% confidence interval) and include estimated systematic errors. A comparison of the present work with previously reported rate coefficients for the CH3 CO + Cl2 reaction is presented. C 2009 Wiley Periodicals, Inc.∗ Int J Chem Kinet 41: 543–553, 2009