The products, kinetics, and mechanism of the reaction Cl + butanone have been measured by UV irradiation of Cl-2/butanone/N-2 (O-2) mixtures using either GC or FTIR analysis. In the absence of O-2, the products are 1-, 3-, and 4-chlorobutanone with yields of 3.1%, 76%, and 22.5%, respectively. As the temperature is increased, the yields of 1-and 4-chlorobutanone increase relative to the 3-chlorobutanone yield. On the basis of these increases, the activation energies for hydrogen abstraction at the 1 and 4 positions are determined to be 1800 (+/-300) and 470 (+300, -150) cal mol(-1) relative to abstraction at the 3 position. In the presence of 400 ppm of O-2 with 700-900 ppm of Cl-2 at 297 K, the yields of 1- and 3-chlorobutanone decrease dramatically from 3.1% to 0.25% and from 76% to 2%, respectively, while the 4-chlorobutanone decreases only slightly from 22.5% to 18.5%. The observed oxygenated species are acetaldehyde (52%), butanedione (11%), and propionyl chloride (2.5%). Increasing the temperature to 400 K (O-2 = 500 ppm) suppresses these oxygenated products and 1- and 3-chlorobutanone again become the primary products, indicating that the 02 addition reaction to the 1- and 3-butanonyl radicals is becoming reversible. At 500 K and very high O-2 mole fraction (170 000 ppm), a new product channel opens which forms a substantial yield (similar to 20%) of methylvinylketone. Computer modeling of the product yields has been performed to gain an understanding of the overall reaction mechanism in the presence and absence of O-2. The reaction of chlorine atoms with butanone proceeds with a rate constant of 4.0 ( +/-0.4) x 10(-11) cm(3) molecule(-1) s(-1) independent of temperature over the range 297-475 K (E-a = 0 +/- 200 cal mol(-1)). Rate constant ratios of k(CH2C(O)C2H5 + Cl-2)/k(CH2C(O)C2H5 + O-2) = 0.027 +/- 0.008, k(CH3C(O)CHCH3 + Cl-2)/ k(CH3C(O)CHCH3 + O-2) = 0.0113 +/- 0.0011, and k(CH3C(O)CH2CH2 + Cl-2)/ k(CH3C(O)CH2CH2 + O-2) = 1.52 +/- 0.32 were determined at 297 K in 800-950 Torr of N-2 diluent. In 700-900 Torr of N-2/O-2 diluent, the major fate of the alkoxy radicals CH3C(O)CH(O)CH3 and OCH2C(O)C2H5 is decomposition to give CH3C(O) radicals and CH3CHO and HCHO and C(O)C2H5 radicals, respectively.