The gas phase reactions of O3 with ethene, isoprene, and a series of monoterpenes have been investigated at 296¿2 K and atmospheric pressure of air in the presence of cyclohexane at concentrations sufficient to essentially totally scavenge any OH radicals formed. The expected products of the OH radical-initiated reaction of cyclohexane, cyclohexanone and cyclohexanol, were observed in all cases. From a knowledge of the chemistry of cyclohexane in these reaction systems and as a result of cyclohexanone and cyclohexanol formation yield data obtained in subsidiary experiments, the formation yields of OH radicals in these O3-alkene reactions were derived. The OH radical formation yields obtained from the gas phase reactions of O3 with alkenes were ethene, 0.12; isoprene, 0.27; camphene, ≤0.18; 3-carene, 1.06; limonene, 0.86; myrcene, 1.15; ocimene (a cis-, trans-mixture), 0.63; β-phellandrene, 0.14; α-pinene, 0.85; β-pinene, 0.35; sabinene, 0.26; and terpinolene, 1.03, all with estimated overall uncertainities of a factor of ~1.5. For α-pinene the effect of varying the water vapor concentration was investigated and no change in the OH radical formation yield was observed over the range of (2.5-24)¿1016 molecules cm-3 of water vapor. The experimental conditions were such that formation of OH radicals from HO2 radicals was of minor importance, and the OH radical formation yields given above refer to direct formation of OH radicals and not HO2 radicals. ¿ American Geophysical Union 1992