The daytime oxidation of α-pinene in the presence of natural sunlight and oxides of nitrogen, NOx, was studied in a large outdoor smog chamber using gas chromatograph mass spectrometry as method of identification and quantification. A Scanning Mobility Particle Sizer system (3936, TSA) and a Condensation Particle Counter (3025A, TSA) were used to study the secondary organic aerosol formation, and a filter pack/denuder sampling system was used for simultaneously collecting gas phase and particle phase products for analysis. A gas chromatograph coupled to an electron impact mass spectrometry (GC-EIMS) method is described for the identification and quantification of gas and aerosol products. The sensitivity of the method was sufficient to produce good quality mass spectra over the range of concentrations used in this study, and compares very well with classical methods based on derivatization and liquid chromatography-ion trap mass spectroscopy methods. Mass balances for gaseous and aerosol reaction products are reported over the course of the reaction. More than 16 products were identified and quantified in this study. On average, measured gas and particle phase products accounted for ~54% to ~71% of the carbon reacted α-pinene. Measurements show that 10-hydroxypinonic acid, 10-hydroxypinonaldehyde, 4-oxopinonic acid, and 10-oxopinonic acid are observed in the early stage in the aerosol phase and may play an important role in the early formation of secondary aerosols. ¿ 2001 American Geophysical Union