The kinetics of the gas-phase reactions of O₃ with a series of alkenes and two α,β-unsaturated ketones have been investigated at atmospheric pressure (ca. 740 torr) of air and 296 ± 2 K, using a relative rate method in the presence of sufficient cyclohexane to scavenge OH radicals generated in these reactions. Combined with our previous relative rate measurements (Int. J. Chem. Kinet., 24, 803 (1992)), the rate constants obtained relative to k(O₃ + propene) = 1.00 were: 3-penten-2-one, 3.62 ± 0.16; 2-cyclohexen-1-one, <0.19; bicyclo[2.2.2]-2-octene, 7.44 ± 0.48; 1,3-cycloheptadiene, 16.1 ± 1.1; cycloheptene, 23.7 ± 1.6; 1,3-cyclohexadiene, 134 ± 13; bicyclo[2.2.1]-2-heptene, 170 ± 16; and bicyclo[2.2.1]-2,5-heptadiene, 390 ± 36. The resulting rate constants then lead to a self-consistent set of room temperature data for the reactions of O₃ with these alkenes and α,β-unsaturated ketones. These rate constants are compared with the literature data, and the effects of ring size discussed. © 1994 John Wiley & Sons, Inc.