Using relative rate techniques, rate constants for the gas phase reactions of OH radicals and NO3 radicals with the sesquiterpenes α-cedrene, α-copaene, β-caryophyllene, α-humulene, and longifolene have been measured at 296¿2 K and atmospheric pressure of air. The rate constants obtained (in units of 10-11 cm3 molecule-1 s-1) for the OH radical and NO3 radical reactions, respectively, are α-cedrene, 6.7¿1.4 and 0.82¿0.30; α-copaene, 9.0¿1.9 and 1.6¿0.6; β-caryophyllene, 20+5-9 and 1.9¿0.8; α-humulene, 29+7-10 and 3.5¿1.3; and longifolene, 4.7¿1.0 and 0.068¿0.021, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference compounds. These rate constants and those for the corresponding O3 reactions are combined with estimated ambient lower tropospheric concentrations of OH radicals, NO3 radicals, and O3 to allow the lifetimes of these sesquiterpenes with respect to these chemical loss processes to be calculated. The sesquiterpenes studied are all reactive, with calculated overall lifetimes of a few hours or less. In particular, β-caryophyllene and α-humulene are highly reactive toward O3 and NO3 radicals, with calculated lifetimes due to these reactions of 1--2 min, and β-carophyllene and α-humulene are therefore not expected to be present in ambient air at measurable concentrations. ¿ American Geophysical Union 1995