The initial products of the tropospheric oxidation of a representative mixture of hydrocarbons have been studied theoretically, using a detailed chemical mechanism involving 26 alkanes (C1--C8), alkenes (ethylene, propylene), aromatic hydrocarbons (benzene, toluene, o-, m-, and p-xylenes), and two terpenes (isoprene, α-pinene). For an NOx-rich atmosphere the oxidation of the alkanes in the mixture leads to three main classes of initial products: aldehydes (34%), ketones (45%), and alkyl nitrates (21%). Both the &ggr; -hydroxy-aldehydes (19%) and the &ggr;-hydroxy-ketones (19% of all alkane initial products) are major products. For NOx-free conditions the major products of the alkane oxidation are expected to be the alkyl hydroperoxides. For NOx-rich conditions the aldehydes dominate the alkene oxidation products (80%), with smaller amounts of α-hydroxy-carbonyl compounds (9%) and organic acids (4%). The major products of the aromatic hydrocarbons in the mixture are α-dicarbonyls (24%), unsaturated &ggr;-dicarbonyls (22%), phenols ( <49%), aromatic aldehydes (3%), and aromatic nitrates (2%). The expected product distribution and lifetime data suggest that each class of hydrocarbon has certain unique products which should serve as useful indicators of the history of an aged air mass. It is concluded that the alkyl nitrates and the &ggr;-hydroxy-alkyl nitrates may contribute to the large, and as yet unidentified, fraction of the active nitrogen species noted in NOy determinations in the troposphere by Fahey et al. (1986). ¿American Geophysical Union 1987