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 present that is converted to aerosol, varied from 0 to 5.3% for α‐pinene, depending on the initial 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 period 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.