Hydroxyl radical reactions play an important role in the removal of monoterpenes from the troposphere during daytime hours, but there are few data available to date concerning the products of these reactions. From the gas-phase reactions of β-pinene and d-limonene with the OH radical in the presence of NOx, the predicted products, 6,6-dimethylbicyclo<3.1.1>-heptan-2-one and 4-acetyl-1-methylcyclohexene, respectively, were observed with formation yields of 30.0¿4.5% and 17.4¿2.8%. On the basis of mass spectral data a number of ketone and keto-aldehyde products were tentatively identified in irradiated CH3ONO--NO--air--monoterpene mistures of d-limonene, α-pinene, Δ3-carene, sabinene, and terpinolene. The estimated formation yields of the observed products range from a low of ~29% for α-pinene to a high of ~45% for d-limonene. No significant products were observed from the OH radical-initiated reaction of myrcene. Certain of these monoterpene reaction products, such as the 6,6-dimethylbicyclo<3.1.1> heptan-2-one observed from β-pinene, are less reactive than the parent monoterpene and are formed in sufficient yield that they may serve as markers for the monoterpene emissions. These gas-phase products should be amenable to analysis by the techniques used for ambient air sampling of monoterpenes. ¿ American Geophysical Union 1990