Catalysis of the electroreduction of H2O2 by the polyoxometalate P2Mo18O626- is described. The presence of polyoxometalate in solution leads to electrocatalytic currents at potentials as positive as + 0.35 V versus Ag/AgCl. Modification of the surface of a glassy carbon electrode by adsorption of P2Mo18O626- results in the formation of an anionic monolayer showing catalytic redox activity at the same unusual potential. The currents of H2O2 electroreduction can be greatly enhanced by coating the electrode with multiple layers of polyoxometalate and a charge balancing cation. A possible catalytic mechanism is proposed. (C) 2000 Elsevier Science Ltd. All rights reserved.