Studies on microelectrodes prepared by electrochemical etching show that noble metals catalytically active towards oxygen reduction like Pt, Pd and Ag are not as inert at high temperature as commonly assumed. The results show that the morphology of the electrode and the reaction zone changes during polarization. Electrode material migrates from the electrode surface and deposits on the YSZ surface forming dendritic structures. Au electrodes, which are catalytically poor towards oxygen reduction, seem in comparison to be inert. The transport mechanism for the electrode material migration is studied by comparing the electrochemical responses and physical properties of the different noble metal electrodes. Also heat treatment of migrated electrode material experiments and polarization of geometric well defined dense thin 1/2-1 mu m layered Lanthanum-Strontium-Manganite (LSM) and Pt films on YSZ experiments, were conducted in order to distinguish between a surface and gas phase transport mechanism. (C) 2007 Elsevier B.V. All rights reserved.