A 312 cu m (11,018 cu ft) Teflon film outdoor smog chamber was constructed in rural North Carolina. The chamber was operated with natural conditions of solar radiation, temperature and relative humidity which existed at the time of a run. Ninety-two 12-hour runs using propylene and oxides of nitrogen were conducted to assess the performance of the system. A photochemical model, in which only the light intensity magnitude and pattern and the rate of heterogeneous surface reactions were changed, was used to compare the outdoor results with those of three indoor chambers. Good agreement was found in all cases. One-hundred-thirty 12-hour runs were conducted using a simulated urban hydrocarbon mix and oxides of nitrogen. Reduction of the hydrocarbon concentration resulted in reductions of nitrogen dioxide (NO2) maximum concentration and, for large reductions, the daytime NO2 dosage. Other factors investigated included NO2 to NO ratio at constant NOx, effect of slow dilution, and results of extended 24-hour and 36-hour runs.