Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with 4-hexen-3-one, 5-hexen-2-one, and 3-penten-2-one have been determined at 298 +/- 2 K and atmospheric pressure of air. Rate coefficients for the compounds were determined using a relative kinetic technique with different reference compounds. The experiments were performed in a large photoreactor (480 L) using in situ FTIR spectroscopy to monitor the decay of reactants. From the different measurements the following rate coefficients (in units of cm(3) molecule(-1) s(-1)) have been determined: k(1)(OH + 4-hexen-3-one) = (9.04 +/- 2.12) X 10(-11), k(2)(OH + 5-hexen-2-one) = (5.18 +/- 1.27) X 10(-11), k(3) (OH 3-penten-2-one) = (7.22 +/- 1.74) X 10(-11) k(4)(Cl + 4-hexen-3-one) = (3.00 +/- 0.58) X 10(-10) k(5)(Cl + 5-hexen-2-one) = (3.15 +/- 0.50) X 10(-10) and k(6)(Cl + 3-penten-2-one) = (2.53 +/- 0.54) X 10(-10). The reactivity of the double bond in alkenes and unsaturated ketones at 298 K toward addition of OH radicals and Cl atoms are compared and discussed. In addition, a correlation between the reactivity of the unsaturated ketones toward OH radicals and the HOMO of the compounds is presented. On the basis of the kinetic measurements, the tropospheric lifetimes of 4-hexen-3-one, 5-hexen-2-one, and 3-penten-2-one with respect to their reaction with hydroxyl radicals are estimated to be between 2 and 3 h.