The kinetics of the reaction of OH radical with isoprene has been investigated at a total pressure of 1-3 Torr over a temperature range of 240-340 K using the relative rate/discharge flow/mass spectrometry (RR/DF/MS) technique. The reaction of isoprene with OH was found to be independent of pressure over the pressure range of 1-3 Torr at 298 K, and the reaction had reached its high-pressure limit at 1 Torr. However, the rate constant of this reaction is found to positively depend on pressure at 1-3 Torr and 340 K. At 298 K, the rate constant of this reaction was determined to be k1 = (10.4 +/- 1.9) x 10(-11) cm3 molecule(-1) s(-1), which is in good agreement with literature values. The Arrhenius expression for this reaction was determined to be k1 = (2.33 +/- 0.09) x 10(-11) exp[(444 +/- 27)/T] cm3 molecule(-1) s(-1) at 240-340 K. The atmospheric lifetime of isoprene was estimated to be 2.9 h based on the rate constant of isoprene + OH determined at 277 K in the present work.