Morphological modifications occurring on platinum catalysts during operation in a phosphotungstic acid fuel cell were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive atomic X-ray (SEM-EDAX) analyses. An increase of platinum particle size was observed on both anodes and cathodes with the time of operation in fuel cell. This phenomenon was significantly larger on the cathode. The uptake of electrolyte by the catalyst layers increased during cell operation. Concurrently, an increase of cell performance was recorded. On the basis of physicochemical characterizations, the growth mechanism of platinum particles was attributed to a dissolution-redeposition phenomenon involving smaller crystallites. Whereas, the increase of cell performance during the first period of operation is likely due to an increase of the three-phase reaction zone at the electrode-electrolyte interface whose positive contribution to the cell polarization prevails over the effects of platinum crystallite growth.