In a first series of experiments the IrO2 coating was labeled with O-18 (formation of (IrOO)-O-16-O-18) by treatment in (H2O)-O-18. Then, in a second series of experiments, this labeled electrode was used for the electrochemical oxidation of formic acid. The presence of (COO)-O-18-O-16 (m/z=46) during the anodic polarization clearly proves that the IrO2 participated in the oxidation of the formic acid. Analysis of the experimental data showed that at maximum 3.7% of the surface IrO2 participated in the exchange of oxygen between electrogenerated labeled IrO3 ((IrOOO)-O-16-O-16-O-18) and adsorbed formic acid ((HCOOH)-O-16-O-16), producing carbon dioxide ((COO)-O-16-O-18) and (IrO2)-O-16. (C) 2008 The Electrochemical Society.