Reactions of OH with α-pinene and β-pinene were studied for the purpose of obtaining the basic data to estimate the emission rate of CO from the photooxidation of terpenes in the atmosphere. In the presence of NO the main product was pinonaldehyde and 6,6-dimethyl-bicyclo<3.1.1> heptan-2-one from α- and β-pinene, respectively, and the yield was 56¿4 and 79¿8%, respectively. In the absence of NO the yield was remarkably lower, and the yield of organic aerosols was enhanced. For α-pinene 56¿3% was obtained as a yield of aerosols on the carbon number basis. Gross annual emission of CO from the reactions of OH with terpenes was estimated to be 22 Tg C yr-1 (50 Tg CO yr-1) by regarding α-pinene as the representative of terpenes, on the basis of the annual emission rate of terpenes, estimated lifetime of pinenes in the atmosphere, the apparent rate constant for the intermediate to form CO, and the estimated yield of CO from the secondary oxidation of pinonaldehyde. The ultimate yield of CO from the tropospheric oxidation of terpenes (including both ozone and OH reactions) was estimated to be 20% on the carbon number basis, and the total annual emission of CO was evaluated to be 96 Tg C yr-1 (222 Tg CO yr-1). ¿ American Geophysical Union 1991