A single cell of a PEMFC with PtRu/C anode and Pt/C cathode catalysts after an acceleration test in which constant high potential was applied to the cathode was observed by analytical TEM to investigate the structural changes in electrocatalysts and the proton-exchange membrane (PEM) at a micro- and nano-scale. A cross-sectional specimen of a membrane-electrode-assembly (MEA) was successfully prepared for TEM observation by using an ultramicrotome. A thin area of the electrode catalyst layer and the PEM were observed at a nano- and atomic-scale. The interface between the catalyst layer and the PEM was also observed. The results showed that large Pt particles of about 10-100 nm were formed in the PEM after an acceleration test in which a constant potential was applied. This result indicates that Pt dissolved and diffused, and Pt crystals grew in the PEM. The distribution of Pt particles in the PEM depended on the kind of gas supplied for cathode. (c) 2005 Elsevier B.V. All rights reserved.