The products, kinetics, and mechanism of the reaction Cl + 3-pentanone have been measured by UV irradiation of Cl-2/3-pentanone/N-2 (O-2) Mixtures using primarily GC analysis with selected cross checks by FTIR. In the absence of O-2, the products are 1- and 2-chloro-3-pentanone with yields of 21 and 78%, respectively. As the temperature is increased, the yield of 1-chloro-3-pentanone increases modestly relative to the 2-chloro-3-pentanone yield, On the basis of this increase, the activation energy for hydrogen abstraction at the 1 position is determined to be 500 (+/- 500) cal mole(-1) relative to abstraction at the 2 position. In the presence of 500 ppm of O-2 With 900-1000 ppm of Cl-2 at 297 K, the yield of 2-chloro-3-pentanone decreases dramatically from 78 to 2.5%, while the 1-chloro-3-pentanone decreases only modestly from 21 to 17%. The observed oxygenated species are acetaldehyde (59%), 2,3-pentanedione (9%), and propionyl chloride (56%). Increasing the temperature to 420 K (O-2 = 500 ppm) suppresses these oxygenated products, and 2-chloro-3-pentanone again becomes the primary product, indicating that the O-2 addition reaction to the 2-pentanonyl radical has become reversible. At 500 K and 10 000 ppm O-2, a new product channel opens which forms a small yield (similar to 4%) of ethylvinylketone. 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 3-pentanone proceeds with a rate constant of 8.1 (+/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1) independent of temperature over the range of 297-490 K (E-a = 0 +/- 200 cal mole(-1)). Rate constant ratios of k(C2H5C(O)CHCH3 + Cl-2)/ k(C2H5C(O)CHCH3 + O-2) 0.0185 +/- 0.0037 and k(C2H5C(O)CH2CH2 + Cl-2)/ k(C2H5CH2C(O)CH2CH2 + O-2) 2.7 +/- 0.4 were determined at 297 K in 800-950 Torr of N-2/O-2, diluent. In 800-950 Torr of N-2/O-2, diluent, the major fate of the alkoxy radical CH3CH(O)C(O)C2H5 is decomposition to give C2H5C(O) radicals and CH3CHO. These results show that the chemical mechanisms of the 3-pentanone reactions are very similar to those observed for butanone. In addition, the rate constants of the reactions of chlorine atoms with 1-chloro-3-pentanone [3 (+/- 0.6) x 10(-11) over the range of 297-460 K], 2,3-pentanedione [1.4 (+/- 0.3) x 10(-11) at 297 K], and ethylvinylketone [1.9 (+/- 0.4) x 10(-10) over the range of 297-400 K, decreasing rapidly above 400 K] were measured at ambient pressure.