Despite the importance of isoprene hydroxy peroxy radicals (RO2) in the conversion of NO to NO2 and hence in tropospheric ozone formation, no direct experimental studies have been carried out to investigate the kinetics of the RO2-NO reaction. We report the first direct laboratory study of the rate constant for the isoprene RO2-NO reaction using a fast-flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. By directly monitoring the peroxy radical, a rate constant of (9 +/- 3) x 10(-12) cm(3) molecule(-1) s(-1) was determined in the pressure range of 1 to 2 Torr and at a temperature of 298 +/- 2 K. The measured rate constant is compared to the results of a recent theoretical prediction and indirect measurements and implications for the atmosphere are discussed.