Products and mechanisms of the gas-phase reactions of O(3P) atoms and O3 with the cycloalkenes α-pinene and 1,2-dimethyl-1-cyclohexene have been investigated at 296¿2 K and 740 torr total pressure, using gas chromatography and in situ atmospheric pressure ionization tandem mass spectrometry. The O(3P) atom reaction with α-pinene led to the formation of α-pinene oxide and two isomeric C10H16O ketones with yields of 0.77¿0.06, 0.18¿0.03, and 0.06¿0.02, respectively. The O(3P) atom reaction with 1,2-dimethyl-1-cyclohexene produced 1,2-dimethyl-1-cyclohexene oxide with a yield of 0.51¿0.16. Product studies of the O3 reactions in the presence of cyclohexane as an OH radical scavenger showed the formation of α-pinene oxide and pinonaldehyde from α-pinene with yields of 0.021¿0.007 and 0.143¿0.024, respectively, and the formation of 1,2-dimethyl-1-cyclohexene oxide, 5-oxohexanal, and 2,7-octanedione from 1,2-dimethyl-1-cyclohexene with yields of 0.020¿0.006, 0.19¿0.05, and 0.07¿0.02, respectively. No evidence for the formation of O(3P) atoms from either the reactions of α-pinene or 1,2-dimethyl-1-cyclohexene with O3 was obtained (with estimated O(3P) atom yields <0.03 and <0.04, respectively), and the formation of the epoxides is attributed to a direct reaction. An OH radical formation yield from the 1,2-dimethyl-1-cyclohexene reaction of 1.02¿0.16 was measured using 2-butanol to scavenge the OH radicals and form 2-butanone. Experiments to elucidate the formation route(s) to 2,7-octanedione and pinonaldehyde were carried out, and it is postulated that these carbonyl products are formed from reactions of the thermalized biradicals with water vapor. ¿ 1998 American Geophysical Union