The formation yields of acetone from the gas-phase reactions of the OH radical (in the presence of NO) and O3 with a series of monoterpenes have been measured at room temperature and atmospheric pressure of air. The acetone formation yields ranged from <2--3% for the OH radical reaction with limonene and the O3 reactions with limonene and α-phellandrene to 50% for the O3 reaction with terpinolene. Combining these acetone formation yields with literature estimates of emission rates of monoterpenes from vegetation leads to an estimate of acetone formation from the atmospheric photooxidation of monoterpenes of ~10--11 Tg yr-1 globally, a significant fraction of the global acetone source strength of 40--60 Tg yr-1. Reaction mechanisms leading to acetone formation from these monoterpene reactions are discussed. ¿ 1999 American Geophysical Union