The electrolytic decomposition of an amaranth dyestuff solution using several combinations of electrodes with diamond and platinum is reported. It is observed that a portion of the amaranth is decomposed on the cathode surface while the other portion is decomposed to lower molecular weight components on the anode surface. The decolourizing rate is higher at diamond electrodes used as the anode and the cathode than with other combinations. This electrode combination also shows a rapid decrease in total organic carbon concentration. Acetic acid and oxalic acid are detected as the intermediate substances, and CO2 gas is generated as a final product corresponding to the decrease in the oxalic acid concentration.