We have investigated products formed from the gas-phase reactions of OH radicals and O3 (at 296 ¿ 2 K and 740 Torr total pressure of purified air) with the biogenic organic compounds myrcene and ocimene, using gas chromatographic analyses. In addition to the formation of acetone the OH radical reaction with myrcene in the presence of NO resulted in the formation of 4-vinyl-4-pentenal in 19 ¿ 4% yield, while the corresponding ocimene reaction showed no formation of the analogous product 4-methyl-3,5-hexadienal (<2% yield). The O3 reactions with myrcene and ocimene in the presence of an OH radical scavenger formed acetone (21 ¿ 3%) and 4-vinyl-4-pentenal (70 ¿ 13%) from myrene, and acetone (24 ¿ 4%) and 4-methyl-3,5-hexadienal (33 ¿ 6%) from ocimene. Reaction pathways leading to these products are discussed.