The photocatalytic degradation of aqueous solution of azure-B over manganese dioxide has been carried out. The photocatalytic reaction has been studied spectrophotometrically by observing absorbance at different time intervals. The effect of variation of various parameters such as concentration of azure-B, pH, amount of semiconductor and intensity of light on the rate of reaction has been observed. The photocatalytic degradation of azure-B follows first order kinetics. A tentative mechanism for the reaction has been proposed.