The mechanism of the OH-initiated oxidation of isoprene has been studied at 300 K and 100 and 150 Torr total pressure using a turbulent flow technique coupled with laser-induced fluorescence detection of the OH radical. The rate constant for the reaction of OH with isoprene was measured to be (10.8 ¿ 0.5) ¿ 10-11 cm3 molecule-1 s-1 at 150 Torr total pressure and independent of pressure between 100 and 150 Torr, in excellent agreement with most previous absolute measurements. In the presence of O2 and NO, propagation of OH radicals and loss of OH through radical termination resulting from the formation of organic nitrates were measured at 150 Torr total pressure and compared to simulations of the kinetics of this reaction system. The results of these experiments are consistent with an overall rate constant of (1.1 ¿ 0.8) ¿ 10-11 cm3 molecule-1 s-1 for the reaction of NO with isoprene-based hydroxyalkyl peroxy radicals, with branching ratios of 0.85 ¿ 0.10 for the bimolecular channel (oxidation of NO to NO2) and 0.15 ¿ 0.10 for the termolecular channel (formation of organic nitrates). Although the organic nitrate yield reported here is the result of indirect measurements, it suggests that isoprene may be a more significant sink of NOx than previously estimated.