Combining non-destructive, identical location-transmission electron microscopy (IL-TEM) with rotating disk electrode (RDE) measurements, the influence of different treatment procedures on the catalytic activity of carbon supported Pt nanoparticles is probed. IL-TEM shows that the treatment of the catalyst has only minor influence on its structure or the particle shape and size; in particular no treatment induced particle agglomeration is observed. At the same time, both CO stripping and CO bulk measurements are significantly influenced by the electrochemical treatment. In consistence with previous studies this can be explained by the removal of defects in the CO adlayer structure while cycling in CO saturated solution. In contrast, however, it is demonstrated that CO annealing has no impact on the oxygen reduction reaction in the mixed kinetic-diffusion control potential region. (C) 2009 Elsevier Ltd. All rights reserved.