Sulfur dioxide, a reducing agent found in urban air, might be expected to react with the oxidizing atmosphere produced by photochemical smog. It does not, however, react directly with either ozone or nitrogen dioxide in air although these reactions can occur in solution or on surfaces. However, sulfur dioxide does react with other, less well-identified oxidants which are formed during the photochemical smog reaction process. One mechanism involves the reaction of SO2 with NO3 (or N2O5) formed as a result of the reaction of NO2 with O3. The interactions of SO2 with photochemical smog were investigated in environmental chambers. A regression analysis, carried out on the data from 23 chamber experiments with 1-butene, indicated that the effects of SO2 on oxidant production depend on the concentrations of water vapor, initial nitrogen dioxide, and sulfur dioxide. The effect also depends on the type of hydrocarbon. Sulfur dioxide was found to reduce the maximum oxidant obtained from 1-butene, 1-heptene, and 2,2,4-trimethylpentane, but to increase the oxidant obtained from toluene. In all cases, however, the maximum NO2 concentration was lower when SO2 was present. The reaction mechanisms involved in producing these effects are discussed.