Roles of Secondary Ion Mass Spectrometry and the local thermodynamic equilibrium approximation are discussed in understanding high temperature electrochemical processes. Fundamental issues are first considered on metal dense electrode (Pt/Au) and oxide electrolyte (YSZ/GDC) systems; the so-called three phase boundary (TPB) mechanism is dominant for YSZ systems, whereas for the GDC systems, a new proton/water mechanism is dominant; protons play an important role in a such way that the cathodic reaction of protons to form water vapors takes place at TPBs, while evaporated water vapors are transported on the GDC surface; oxide ions are transported to the anode and protons are circulated to the TPBs of the cathode; another effect of water vapors is to enhance oxygen isotope exchange reactions over a whole area of surface and a resulting flow of O-18 inside GDC masks the charge transfer process in the SIMS analyses. A local proton flow in the proton/water mechanism provides a drastic change in the distribution of chemical potential of water vapors under the dense electrodes, leading to the pore formation or the non-homogenized cation distribution. Concerning industrial issues, two degradation topics are analyzed on drastic changes in chemical potential distribution caused by electrode reactions or by non-uniform flows of oxide ions inside electrolyte. (C) 2012 Elsevier B.V. All rights reserved.