Secondary organic aerosol (SOA) formation from the photooxidation of <I>m</I>-xylene, toluene, and benzene is investigated in the Caltech environmental chambers. Experiments are performed under two limiting NO<sub>x</sub> conditions; under high-NO<sub>x</sub> conditions the peroxy radicals (RO<sub>2</sub>) react only with NO, while under low-NO<sub>x</sub> conditions they react only with HO<sub>2</sub>. For all three aromatics studied (<I>m</I>-xylene, toluene, and benzene), the SOA yields (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) under low-NO<sub>x</sub> conditions substantially exceed those under high-NO<sub>x</sub> conditions, suggesting the importance of peroxy radical chemistry in SOA formation. Under low-NO<sub>x</sub> conditions, the SOA yields for <I>m</I>-xylene, toluene, and benzene are constant (36%, 30%, and 37%, respectively), indicating that the SOA formed is essentially nonvolatile. Under high-NO<sub>x</sub> conditions, aerosol growth occurs essentially immediately, even when NO concentration is high. The SOA yield curves exhibit behavior similar to that observed by Odum et al. (1996, 1997a, b), although the values are somewhat higher than in the earlier study. The yields measured under high-NO<sub>x</sub> conditions are higher than previous measurements, suggesting a "rate effect" in SOA formation, in which SOA yields are higher when the oxidation rate is faster. Experiments carried out in the presence of acidic seed aerosol reveal no change of SOA yields from the aromatics as compared with those using neutral seed aerosol.