In this work, anodic oxidation of C.I. direct blue 21 was carried out in an electrochemical reactor at a constant temperature of 20°C. The technique was found successful in removing the color of the dye from aqueous solutions containing 0.5 M sodium sulfate. Variables studied were: initial dye concentration, solution flow rate to the reactor, anode area, current density and time duration of anodic oxidation. It was found that the rate of dye removal increased with increasing current density, duration of anodic oxidation and solution flow rate. The mass transfer coefficient was found to increase with increasing reactor current density, decreasing the area of the anode and decreasing the initial dye concentration.