The photocatalytic degradation of eriochrome black T was investigated using ammonium phosphomolybdate semiconductor, under visible light irradiation. Ammonium phosphomolybdate semiconductor was used as a photocatalyst for degradation of eriochrome black T under different conditions of pH, amount of semiconductor, dye concentration and light intensity. The degradation rate was affected by all these parameters. The degradation of dye followed pseudo-first order kinetics. A maximum degradation of 93.9 % was achieved after irradiation time of 5 hours. The mechanism of photocatalysis has been discussed.