A comparative study of the activity of different homogeneous photochemical and heterogeneous photocatalytic systems in oxidation of methylviologen in aqueous solutions is done. The photochemical systems used contained hydrogen peroxide or a mixture of hydrogen peroxide and potassium ferrioxalate or iron(III) chloride additives. The photocatalytic systems contained TiO2 with or without hydrogen peroxide additives. The system activities were compared by the initial quantum yield of the process, the initial rate of the fall in the optical density of the organic component as well as the kinetics of carbon dioxide evolution during complete mineralization of methylviologen molecules. The homogeneous photochemical systems with pH=3 are shown to be much more active than the heterogeneous photocatalytic systems. As pH grows to 10, the photocatalytic system activity significantly increases in initial oxidation stage; however, in final stage the process becomes slow. At optimal pH=3 the activity of the homogeneous system is twice as high as the activity of the photocatalytic system at its optimal pH=10.