This thesis is part of the durability of PEMFC and is particularly interested in the degradation of electrocatalysts of type Pt / C that are used within their electrodes. The objective was to determine mechanisms responsible for their degradation, by combining experiments in electrochemical cell with three electrodes, transmission electron microscopy and scanning, and mass spectrometry (DEMS). Initially we have shown that the characterization methods used can influence the degradation observed. Thus, the morphology of Pt nanocatalysts / C may be altered by the TEM observation itself, as does the sequence of measurements by CO-stripping.