The kinetics of reactions of alpha-pinene and beta-pinene with hydroxyl radicals (OH) has been investigated at 1-8 Torr and 240-340 K using the relative rate/discharge flow/mass spectrometry (RR/DF/MS) technique. Our kinetic results indicate that at 298 K the rate constant of the reactions of alpha-pinene and beta-pinene with hydroxyl radicals has little pressure dependence over the 1-8 Torr pressure range, suggesting that the high pressure limit of these reactions has been reached at 1 Torr. The rate constant of these reactions was found to negatively depend on the temperature at 240-340 K, which is consistent with previous investigations using different techniques. The Arrhenius equation for alpha-pinene and beta-pinene with hydroxyl radical was determined to be k(alpha-pinene) = (1.21 +/- 0.20) x 10(-11)exp[(441 +/- 46)/T] cm(3) molecule(-1) s(-1) and k(beta-pinene) = (1.65 +/- 0.10) x 10(-11)exp[(470 +/- 17)/T] cm(3) molecule(-1) s(-1) respectively. Using the rate constant determined at 277 K in this work and the average global hydroxyl radical concentration, the atmospheric lifetime of alpha-pinene and beta-pinene was estimated to be 5.8 and 3.8 h, respectively.