NiS-ZnS in molar ratio 1 : 2 was prepared using coprecipitation method. It photocatalytic activity was evaluated using alizarin red S as a model dye. The dependence of the rate of degradation on the amount of catalyst, pH, concentration of dye and light intensity has been studied. A comparative study on the photocatalytic activities of NiS-ZnS with pure ZnS was also made. A tentative mechanism for the photocatalytic degradation of alizarin red S has also been proposed.