The reaction kinetics of atomic chlorine with a series of biogenic hydrocarbons, including the two enantiomers of ␣-pinene, were studied at 298 K and 1 atm pressure using a relative rate technique. The simultaneous losses of the biogenic of interest and a reference compound, either n-nonane or n-butane, were followed using gas chromatography with flame ionization detection as a function of the extent of photolysis of a chlorine atom precursor. Thionyl chloride, trichloroacetyl chloride or in a few trials, acetyl chloride, were photolyzed at 254 nm to generate chlorine atoms, since molecular chlorine reacted in the dark with these organics. The relative rate constants for ethane and isoprene determined relative to n-butane using SOCl 2 and CCl 3 COCl were compared to those determined using Cl 2 to check for possible artifacts. The average relative rate constants for ethane and isoprene (both relative to nbutane) using these new sources are (0.281 Ϯ 0.021) and (2.49 Ϯ 0.39) (Ϯ2 ) respectively, within experimental error of those measured using Cl 2 as the chlorine atom source. The relative rate constants averaged over all sources including Cl 2 are (0.277 Ϯ 0.025) for ethane and (2.42 Ϯ 0.45) for isoprene. The ratios of rate constants for the chlorine atom reactions with the biogenics with formula C 10 H 16 relative to n-nonane were as follows: (R)-␣-pinene (0.991 Ϯ 0.264); (S)-␣-pinene (0.946 Ϯ 0.240); -pinene (1.09 Ϯ 0.30); (R)-limonene (1.33 Ϯ 0.15); myrcene (1.36 Ϯ 0.31); 3-carene (1.16 Ϯ 0.23). That for p-cymene, C 10 H 14 , is (0.433 Ϯ 0.072). Taking k(Cl ϩ n-nonane) ϭ (4.82 Ϯ 0.14) ϫ 10 Ϫ10 cm 3 molecule Ϫ1 s Ϫ1 , the absolute rate constants (in units of 10 Ϫ10 cm 3 molecule Ϫ1 s Ϫ1 ) are: (R)-␣-pinene (4.8 Ϯ 1.3); (S)-␣-pinene (4.6 Ϯ 1.2); pinene (5.3 Ϯ 1.5); limonene (6.4 Ϯ 0.8); myrcene (6.6 Ϯ 1.5); 3-carene (5.6 Ϯ 1.3); p-cymene (2.1 Ϯ 0.4). (All errors are Ϯ 2 ). Although abstraction was not measured directly in this study, it is likely a significant contributor to the overall reactions of the C 10 H 16 biogenics. The rate constant for the reaction of the aromatic compound p-cymene is within experimental error of that predicted from the sum of reaction with toluene plus the isopropyl substituent. A limited number of experiments for methyl vinyl ketone in N 2 using CCl 3 COCl as the chlorine atom source and nonane as the reference compound gave a relative rate constant of (0.422 Ϯ 0.034), corresponding to an absolute rate constant of (2.0 Ϯ 0.2) ϫ 10 Ϫ10 cm 3 molecule Ϫ1 s Ϫ1 . Based on these rate constants, the lifetimes of these biogenics at dawn with respect to reaction with chlorine atoms are expected to be comparable to reaction with OH. Thus, loss of these biogenics by reaction with atomic chlorine must be taken into account in coastal regions in addition to their reactions with OH, O 3 and at night, NO 3 .