The reactions of α-pinene and of its main oxidation product, pinonaldehyde (3-acetyl-2,2-dimethyl-cyclobutyl-ethanal), with OH radicals have been studied in the laboratory using Fourier transform infrared spectroscopy for real-time monitoring of the gas-phase chemical species and a Scanning Mobility Particle Sizer system (3071 A, TSI) for the study of the secondary aerosol formation. All gas-phase molar yields were quantified using calibrated reference of the pure compound, except for the nitrates products. The results were: for the α-pinene experiments in the presence of NOx, pinonaldehyde, (87¿20)%; total nitrates (18¿9)%; formaldehyde, (23¿9)%; acetone (9¿6)%; for the α-pinene experiments in the absence of NOx: pinonaldehyde, (37¿7)%; formaldehyde, (8¿1)%; acetone, (7¿2)%; for the pinonaldehyde experiments in the presence of NO, formaldehyde (152¿56)% and acetone (15¿7)%. The aerosol measurements showed that the condensed products accounted for the missing carbon in the gas-phase balance. The partitioning of the products into the condensed phase was found to be potentially significant under experimental conditions but less than 10% for initial α-pinene concentrations lower than 1013 molecule cm-3 and hence negligible under atmospheric conditions in the absence of aerosol seeds. On the basis of these results a comprehensive mechanism for the gas-phase reaction of α-pinene with OH in the presence of NOx has been proposed, including quantitative values for all the involved branching ratios. ¿ 1999 American Geophysical Union