The reaction of chlorine atoms with isoprene (2-methyl-1,3-butadiene) may be important in the marine boundary layer. The kinetics and mechanisms of this reaction have been studied at 760 Torr and 298 K using the photolysis of Cl-2 as the chlorine atom source. Product studies in air were carried out using 40 m path length FTIR spectrometry. Rate constants were determined in either air or N-2 using a relative rate technique and GC-FID to follow the decay of isoprene relative to it-butane. For comparison, the kinetics of the chlorine atom reactions with isoprene-d(8), 1,3-butadiene, and 1,3-butadiene-d(6) were also studied at 1 atm in air or N-2 at 298 K. The rate constant ratios determined with n-butane as the reference compound are as follows : isoprene (2.16 +/- 0.17); isoprene-d(8) (1.79 +/- 0.22), 1,3-butadiene (2.01 +/- 0.11); 1,3-butadiene-d(6) (1.85 +/- 0.10) (+/- 2 sigma)). The corresponding absolute rate constants, based on a value for n-butane of 2.11 +/- 0.18 x 10(-10) cm(3) molecule(-1) s(-1), are as follows : isoprene (4.6 +/- 0.5); isoprene-d(8) (3.8 +/- 0.6); 1,3-butadiene (4.2 +/- 0.4); 1,3-butadiene-d(6) (3.9 +/- 0.4) (+/- 2 sigma, all in units of 10(-10) cm(3) molecule(-1) s(-1). Both the kinetics studies and the HCl yields determined using FTIR show that the net abstraction of an allylic hydrogen is a small but significant fraction, 15 +/- 4% (+/- 2 sigma), of the overall reaction at 1 atm. The major organic products appear to be unsaturated chlorine-containing carbonyl compounds which remain unidentified. The atmospheric implications are discussed.