The electrodes for DEMS were deposited from a water base suspension containing approximately 2 g of the catalyst per liter on a Ti mesh (open area 20%, Goodfellow). The deposition aimed at obtaining the surface coverage of about 1-2 mg/cm 2 . The deposited layers were later stabilized by annealing the electrodes for 4 hours at 400 ºC in air. All electrochemical experiments were carried out in 0.1 M HClO4 dissolved in H218O (98 %, Medical Isotopes Inc., USA) in a home-made Kel-F single compartment cells described previously in . The residual 16 O fractions before and after electrochemical experiments coincided within the margin of the experimental error and ranged between 0.02 and 0.06. The experiments were done in a three electrode arrangement with Pt auxiliary and an Ag/AgCl (sat. KCl) reference electrode placed outside the cell. All potentials in the paper are quoted in RHE scale. The potential control in the experiments was achieved using PAR 263A potentiostat. The DEMS measurements were done on an experimental station composed of Prisma TM QMS200 quadrupole mass spectrometer (Balzers) connected to TSU071E turbomolecular drag pumping station (Balzers).