INTRODUCTION The objective of this program is to determine the nature and extent of the interaction of sulfur dioxide with the photo-induced reactions of hydrocarbons and nitrogen oxides in air. During the first year@ reactions of the system S02/l-butene/NO/air were examined. (1) ** Experiments were conducted to measure the rates of formation and depletion of the principal species and the rate of increase in light scattering due to aerosol formation. SO2 concentration and relative humidity ^.'ere the (2) principal variables. During the second year @ the progiam was extended to include additional parameters such as variations in the type of hydrocarbon and the hvdrocarbon/NOx ratio. The effects of added particulates on the smog reaction sequence as well as the reactions of SO2in the absence of hydrocarbons with NO@ NO2@ 03@ and particulates were investigated.@ The third-year study@ discussed in this report@ investigated the following: (1) the reactivity of S02with smog from 18 different hydrocarbons and from auto exhaust@ (2) the effect of variations in the amount of N02 in the nitrogen oxides@ (3) the effect of stirring and high humidity on aerc^ol formation@ and (4) ozone and nonozone oxidant formation. **Reference are given on page42.