We report laboratory kinetic studies of isoprene reactions initiated by the hydroxyl radical OH, using a turbulent flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. The rate constants for the reaction of isoprene with OH have been measured in the pressure range of 70 to 120 torr at 298¿2 K and are found to be independent of pressure with an averaged value of (10.1¿0.8)¿10-11 cm3 molecule-1 s-1. The error limit given is within 1 standard deviation; a systematic error is estimated to be ¿15%. We also describe direct observation of the OH-isoprene adduct based on ion-molecule reactions by using the CIMS method. The formation of the OH-isoprene adduct was used to extract the rate constant between OH and isoprene; within the uncertainty of the experiments the results were consistent with those obtained from the observed disappearance of OH. By monitoring the formation of the OH-isoprene adduct in the presence of oxygen molecules, an overall rate constant between OH-isoprene adduct and O2 has been first determined, with an averaged value of (2.8¿0.7)¿10-15 cm3molecule-1 s-1 at 76 torr and an estimated systematic error of ¿50%. Atmospheric implications of the present results to the photochemical oxidation of isoprene are discussed. ¿ 2000 American Geophysical Union