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The rate constants for the C1 atom reaction with three branched ketones have been measured at 298 ± 2 K and 760 Tort using the relative rate method in the absence of NO.The rate constants values obtained (in units of 10-10 cm3/(molecule·sec)) are:k(2-methyl-3-pentanone) =1.07 ± 0.26,k(3-methyl-2-pentanone) =1.21 ± 0.26,and k(4-methyl-2-pentanone) =1.35 ± 0.27.Combining the chemical kinetic data obtained by this study with those reported for other ketones,a revised Structure Activity Relationship (SAR) parameter and R group reactivity (kR) of R(O)R’and CHx (x =1,2,3) group reactivity (kCHx) toward Cl atoms were proposed.In addition,the products from the three reactions in the presence of NO were also identified and quantified by using PTR-ToF-MS and GC-FID,and the yields of the identified products are:acetone (39％ ± 8％) + ethanal (78％ ± 12％),2-butanone (22％ ± 2％) + ethanal (75％ ± 10％) + propanal (14％ ± 1％) and acetone (26％ ± 3％) + 2-methylpropanal (24％ ± 2％),for Cl atoms reaction with 2-methyl-3-pentanone,3-methyl-2-pentanone and 4-methyl-2-pentanone,respectively.Based on the obtained results,the reaction mechanisms of Cl atoms with these three ketones are proposed.