In this study, we made use of structural markers to gain more insights into the structural stability of commercial carbon-supported Pt electrocatalysts under oxidizing, inert and reducing atmospheres. The materials were characterized by electron microscopy and electrochemical techniques in the fresh state and after various aging conditions. The results show that Pt nanoparticles supported on Vulcan XC72 are not immobile but prone to agglomerate during potential sweeping in the presence of hydrogen (H-2). methanol (CH3OH), and carbon monoxide (CO). The migration rate of the Pt crystallites is the largest in CO-containing solution and decreases in the order CO > CH3OH > H-2. We postulate that the morphological changes of the Pt/C nanoparticles are related to the reduction of the oxygen-bearing carbon surface groups strongly interacting with the Pt metal phase. (C) 2012 Elsevier B.V. All rights reserved.