In order to determine the optimal amount of the photocatalyst, a series of experiments with a varied amount of ZnO:AC composites have been conducted. The amount of the catalyst was varied between 10–50 mg/50 ml of the aqueous dye solution. The results of these experiments are shown in Fig. 9. The degree of decolorization of the dye solution increases with increasing amount of the catalyst, and the highest efficiency was attained when the catalyst was 20 mg /50 ml and then decreases. An increase in the efficiency is due to an increase in the number of active sites on ZnO:AC available for the reaction, which in turn increases the rate of radical formation.