Abstract: Minute amount of sulfur dioxide and impurities are always present in the air of industrial area. Under sunlight, smog is formed, scattering light and reducing visibility. This article is to ascertain the effect of rate of photochemical oxidation of sulfur dioxide in air at high relative humidities. During experiment, pure air from a special kind of filter was mixed with sulfur dioxide and stored in a Lucite reactor (7.94 liters), with RS sunlamp shining successively through an infra-red filter and Plexiglas IA on the top of the reactor for 1 to 2 hours. The size of sulfuric acid aerosol particles thus formed was in logarithmic-probability distribution. The particles were impacted at high velocity upon the photographic film which was impregnated with thymol blue indicator after having had its silver salts removed. The amount of sulfuric acid formed during the reaction was determined from measuring the intensity of light passing through the red spot on the film. The experimental results showed that at high relative humidities, the reaction rates were essentially the same as those at low humidities. This indicated that the reaction was chiefly gas-phase, with relatively very small amount of reaction in the liquid phase. Nitrogen dioxide, sodium chloride nuclei and vanadium pentoxide, the impurities usually present in the air of industrial area, showed no effect toward reaction rate. About 300 hours of intense sunlight would be needed to reduce visibility to 1 kilometer by oxidation of the sulfur dioxide to be expected in naturally occurring polluted atmosphere,