An experimental study was conducted to determine the surface area loss of platinum supported on carbon in a 191°C phosphoric acid fuel cell environment and to define the mechanism for this process. The surface area was found to decline rapidly initially, but remains above 20 m2/g for 20,000 hr, the longest time period investigated. A study of the effect of operating potential, transmission electron microscope studies of electrodes, and Pt deposited on a carbon film, and the results of other previous studies indicate that crystallite migration and coalescence are the major mode by which Pt surface area is lost in this situation.