The electrochemical reduction of anthraquinone-2-sulfonate (AQS) has been examined in the presence of an azoic dyestuff (Acid Yellow 9, 4-amino-1,1’-azobenzene-3,4’-disulfonic acid, sodium salt) in alkaline solution. The dianion of the anthrahydroquinone-2-sulfonate (AQS(2-)) formed by cathodic reduction reacts with Acid Yellow 9 to regenerate AQS and yields the reduced dyestuff. The catalytic currents generated due to the chemical reaction between the AQS(2-) and the dyestuff have been used to estimate the rate constant of the reaction.