The OH- and O3-initiated oxidation of five monoterpenes (myrcene, terpinolene, Δ3-carene, α-pinene, and β-pinene) has been studied in environmental chambers equipped with either a Fourier transform infrared spectrometer or a gas chromatography/flame ionization detector system. The OH-oxidation of myrcene and terpinolene is shown to lead to substantial yields of acetone (36 and 39%, respectively), while the acetone yield from the pinene compounds is quite small (4% and ~2%, for α- and β-pinene, respectively). Formaldehyde has been identified as a major product (yields of 20--40%) in the OH-initiated oxidation of all five species. Formic acid was also observed in the OH-initiated oxidation of all five monoterpenes, with yields of 2% from β-pinene and 5--9% from the other species studied. The production of acetone from the reaction of monoterpenes with ozone in the presence of an OH scavenger was measured. The yields of acetone for the O3 reactions were α-pinene, 0.03¿0.01; β-pinene, 0.009¿0.009; Δ3-carene, 0.10¿0.015; myrcene, 0.25¿0.06; and terpinolene, 0.50¿0.06. The mechanism leading to the production of these compounds is discussed, as is the atmospheric relevance of the results. In particular, an estimate of the contribution of monoterpene oxidation to observed atmospheric levels of acetone and formic acid is made. ¿ 2000 American Geophysical Union