Aerosol formation from α-pinene and β-pinene was studied in a series of outdoor smog chamber experiments. Since a previous study focused on β-pinene (Pandis et al., 1991), more attention was given here to α-pinene. The initial hydrocarbon and NOx concentrations ranged from 37 to 582 ppb and 31 to 380 ppb, respectively. The aerosol carbon yield, the fraction of the carbon initially hydrocarbon-to-NOx ratio. Dual-bag experiments demonstrate that α-pinene is more rapidly photooxidized, and produces higher yields of both aerosol and ozone in a given peirod of time than β-pinene, although given sufficient time β-pinene can produce equivalent aerosol yields. Although aerosol formation solely from isoprene photooxidation was found in a previous study to be negligible under ambient conditions, the addition of isoprene to the α-pinene/NOx system leads to an increased aerosol yield through the enhanced photochemical activity generated. ¿American Geophysical Union 1992