A novel, non-destructive transmission electron microscopy technique is introduced, which enables the observation of the identical locations of a catalyst before and after electrochemical treatment (IL-TEM). The significance of this method is exemplified by the analysis of a standard, commercially available carbon supported platinum catalyst. We demonstrate that the observed changes of the catalyst particles are a direct consequence of the applied electrochemical treatment; and that selected catalyst regions are representative for the catalyst as a whole. Different electrochemical treatments were applied in order to discuss the potential of the method for studying processes of catalyst degradation.