Active catalysts for water oxidation to evolve O-2 are required for the construction of artificial photosynthetic devices that are expected to be promising energy-providing systems in the future. The citrate-stabilized IrO2 colloid was self-assembled onto an indium tin oxide (ITO) electrode to form a monolayer of the colloidal IrO2 particles when it was dipped in the colloid solution. The self-assembly could be achieved by a chemical interaction between carboxylate groups on the citrate stabilizer and hydroxyl groups on the ITO surface to form ester bonds. Efficient electrocatalysis for water oxidation was demonstrated using the electrode modified by the self-assembled IrO2 colloid to yield the highest turnover frequency ((2.3-2.5) x 10(4) h(-1)) of IrO2 in the hitherto-reported catalysts for electrochemical water oxidation.