Dilute diesel emission systems were studied in a 600 cu m indoor smog chamber under various conditions likely to be encountered in an urban environment. Aerosol formation and growth in an irradiated diesel emission/SO2/propylene system was primarily attributed to sulfate formation. During irradiated experiments, the rate of SO2 oxidation experienced a maximum of about 5 percent/hr in the period when the O3 concentration was increasing rapidly. Following the peak in the O3 concentration, the rate of SO2 oxidation decreased to about 1 percent/hr. The presence of the primary diesel aerosol appeared to increase the rate of SO2 oxidation and the amount of aerosol sulfate formed as well as minimize the formation of new aerosols in irradiated experiments. In most irradiated experiments, the production of organic and nitrate aerosols represented minimal contributions to aerosol formation and growth. The results of the Ames tests were as follows: (1) UV irradiation produced no significant changes, (2) UV irradiation in conjunction with ozone production decreased mutagenicity and resulted in a change in mutagen type demonstrated by increases in direct acting response over that requiring metabolic activation, (3) dark reaction of the exhaust gas with the aerosols resulted in increased mutagenicity, both with and without metabolic activation.