Rate constants were measured for gas-phase reactions of the hydroxyl radical (OH) with six polycyclic aromatic hydrocarbons (PAH) and four polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). These OH reaction rate constants were determined in helium at about 1 atm over various temperature ranges between 306 and 405 K. The experiments were carried out in a small, heated quartz reaction chamber sampled by on-line mass spectrometry, and OH was produced by the photolysis of O-3 in the presence of H2O. Arrhenius regressions were performed with the rate constants of each compound, and the temperature dependencies were found to be slight to nonexistent. The OH reaction rate constants of PCDD/F were in agreement with those predicted by a structure-reactivity method. The resulting rate constants at 298 K (in units of 10(-12) cm(3) s(-1)) were as follows : naphthalene, 23; acenaphthene, 58; fluorene, 13; phenanthrene, 27; anthracene, 190; fluoranthene, 11; dibenzo-p-dioxin, 12; 2,7-dichlorodibenzo-p-dioxin, 4.4; dibenzofuran, 3.5; and 2,8-dichlorodibenzofuran, 2.2.