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 airq During the first year reactions of the system SO2/l-butene/NO/air were examined. 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 formationo SO2 concentration and relative humidity were the principal variables. During the second year the program was extended to include additional parameters such as variations in the type of hydrocarbon and the hydrocarbon/NOx ratio. The effects of added particulates on the smog reaction-sequence have been investigated. Studies have been made of the reactions of SO2 in the absence of hydrocarbons with NO@ NO2@ O3@ and particulates.