The degradation reactions of isoprene initiated hv chlorine atom were investigates using a fast-flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. The kinetic studies were performed in the pressure range of 5 to 10 Torr and at 298 +/- 2 K. The rate constant for the reaction between Cl and isoprene was measured to be (4.0 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1). Using the CIMS method, we were able to directly detect the products of the Cl-isoprene reaction, HCl and Cl-isoprene adduct. A branching ratio of (17.4 +/- 4.0)% was determined for the HCl formation. We have also measured the rate constant for the reaction of the Cl-isoprene adduct with molecular oxygen to form the beta-chloroalkenylperoxy radicals, with a value of (1.0 +/- 0.3) x 10(-14) cm(3) molecule(-1) s(-1). Atmospheric implications of the present experimental results are discussed.