The effective platinum metal surface area in a real phosphoric acid fuel cell was evaluated by in situ cyclic voltammetry which has been developed in our laboratory. It was found that the in situ electrochemical platinum surface area of a phosphoric acid fuel cell under real operational conditions was around 25 m2/g. It was concluded that this low value came from the isolation of the platinum particles from a carbon support, which was revealed by transmission micrography and the distortion of an in situ cyclic voltammogram. Furthermore, x-ray photoelectron spectroscopy analyses of long-term operated electrodes showed that two or three-fluorines were detached from the polytetrafluoroethylene (PTFE) surface. The timing of PTFE degradation was in good agreement with the change in the in situ EC-MSA.