The kinetics of reactions of 4-methyl-2-pentanone (MIBK) and m-ethyl toluene (MET) with hydroxyl radicals has been studied at a total pressure of 1-8 Torr and 240-340 K using the RR/DF/MS technique. The rate constant for the reaction of MIBK with the hydroxyl radical was found to be essentially pressure-independent in the range of 1-3 Torr. The rate constant for MET reaction with the hydroxyl radical increased with pressure at 1-5 Torr, and a high pressure limit was reached at 5 Torr. At 298 K, rate constants (in cm(3) molecule(-1) s(-1)) of kmisx-FoH(298 K) = (1.25 +/- 0.21) x 10(-1)1 and k(MET+)(OH) (298 K) = (2.05 +/- 0.23) x 10(-11) were determined with n-nonane as the reference compound, while k(M1BK)(+OH) (298 = (1.25 +/- 0.11) x 10(-11) and k(MET+)(OH) (298 K) = (1.88 +/- 0.17) x 10(-11) were obtained with 1,4-dioxane as the reference compound, where k(M1BK)(+OH) was measured at 1 Torr and k(MET+OH) was measured at 5 Torr. The rate constant of MIBK reaction with the hydroxyl radical was found to be negatively dependent on temperature at 240-340 K, with an Arrhenius expression of k MIBK+OH(T) = (2.00 +/- 0.07) x 10(-12) exp[(535 +/- 11)/T] cm(3) molecule(-1) s(-1). The rate constant of MET reaction with the hydroxyl radicals was also found to negatively depend on temperature at 240-340 K but with nonlinear behavior in the Arrhenius plot.