Abstract: Kinetics of the oxidation of sulfur dioxide on Vanadium catalyst used in the sulfuric acid contact process is complicated not only by diffusional processes but also by the change occurring in the catalyst itself. Kinetic studies have been carried out in flow reactors at different temperatures with catalyst granules of different sizes. The effects of mass transfer have been observed. With catalyst size as ordinarily used in industry (5 mm), the reaction rate is much retarded by diffusional processes and is controlled kinetically by outer diffusion when temperature is higher than 500℃. Different rate equations are obtained with different granular sizes of the catalyst. There exists a range of temperature in which the activity of the catalyst changes according to the operating conditions. Direct observation of the catalyst in operation shows that the catalyst is bluish-green in color at 400℃ and becomes yellowish at 500℃. At 450℃ about half of the catalyst bed from the gas inlet end is bluish-green in color and the other half toward the gas exit end yellowish. The proportion of these two colored parts changes with the operating conditions. A lower activity corresponds to more of the catalyst in bluish-green color and a higher activity to more of the catalyst in yellowish. The rate equation obtained changes in form as the catalyst changes in color (the influence of rate of mass transfer not involved). For the foreward reaction at 400℃,For the reverse reaction at 550℃, Below 475℃ the activation energy is 50 Kcal/mol, but above 475℃ it becomes 24 Kcal/mol. The reduction of activation energy is due to change of the catalyst itself but not to the effect of diffusional processes. Examination of the rate equations according to the theory of indicates that the rate determining step is the reaction in the absorbed layer.