Chlorine atoms are important oxidants in the marine boundary layer at mid-latitudes at dawn. Given the high ozone-forming potential of alkenes, understanding the kinetics of their reactions with Cl atoms in coastal areas is important. Using the relative rate technique, the kinetics of the gas phase reactions of atomic chlorine with a series of alkenes, relative to heptane as a reference, have been investigated at (298 ± 3) K and 1 atmosphere in synthetic air and nitrogen. Using 3.97 x 10^-10 cm³ molecule^-1 sec^-1 as the rate constant for the reaction of chlorine atom with heptane, the rate constants, k (in units of 10^-10 cm³ molecule^-1 sec^-1) for the following alkenes were determined: propene (2.64 ± 0.21); isobutene (3.40 ± 0.28); 1-butene (3.38 ± 0.48); cis-2-butene (3.76 ± 0.84); trans-2-butene (3.31 ± 0.47); 2-methyl-1-butene (3.58 ± 0.40); 2-methyl-2-butene (3.95 ± 0.32); 3-methyl-1-butene (3.29 ± 0.36); 2-ethyl-1-butene (3.89 ± 0.41); 1-pentene (3.97 ± 0.36); 3-methyl-1-pentene (3.85 ± 0.35); and cis-4-methyl-2-pentene (4.11 ± 0.55). Cl atom reactions with alkenes is known to occur primarily through the addition to the double bond. From the known rate constant for alkyl hydrogen abstraction, the rate constant of chlorine atom addition to the double bond and that for the abstraction of the allylic hydrogen atoms can be calculated. The rate constant for the allylic hydrogen abstraction is showing to be surprisingly small. The atmospheric implications will be discussed.