Products of the gas phase reactions of the OH radical with α- and β-pinene in the presence of NO have been investigated using gas chromatography and in situ atmospheric pressure ionization mass spectrometry (API-MS). Acetone was identified and quantified by gas chromatography with flame ionization detection and combined gas chromatography-mass spectrometry, with formation yields of 0.110¿0.027 from the α-pinene reaction and 0.085¿0.018 from the β-pinene reaction. Acetone is a first-generation product and may arise after initial H atom abstraction from the tertiary allylic C-H bond at the 1 position and/or after OH radical addition at the 2 and 3 positions. Using API-MS and API-MS/MS analyses, in addition to the formation of pinonaldehyde from α-pinene and nopinone from β-pinene, we have observed the formation of hydroxynitrates, dihydroxynitrates and dihydroxycarbonyl products of molecular weights 215, 231 and 184, respectively, from both α- and β-pinene. Reaction schemes leading to the formation of these multifunctional product species, consistent with our understanding of the atmospheric chemistry of organic peroxy and alkoxy radicals, are presented. ¿ 1998 American Geophysical Union<