- The degradation reactions of isoprene initiated by chlorine atom were investigated 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 2982 K. The rate constant for the reaction between Cl and isoprene was measured to be (4.00.3)10-10 cm3 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 (17.44.0)% 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 -chloroalkenylperoxy radicals, with a value of (1.00.3)10-14 cm3 molecule-1 s-1. Atmospheric implications of the present experimental results are discussed.