Oxygen isotope exchange and time-of-flight secondary-ion mass spectroscopy were used to visualize the oxygen pathway in a silver cathode for solid-oxide fuel cells. (18)O was electrochemically transferred into the electrolyte through the silver cathode and immobilized by quenching. The three-dimensional distributions of (16)O, (18)O, (107)Ag, (109)Ag, and GaO were analyzed by secondary-ion mass spectroscopy. The oxygen pathway was determined as follows: oxygen atoms were placed uniformly on the silver cathode surface, diffused along the silver grain boundaries from the surface to the Ag/electrolyte interface, spread through the entire interface, were ionized, and transferred into the electrolyte. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.045111jes] All rights reserved.