Morphological changes occurring in membrane electrode assemblies (MEAs) were monitored using transmission electron microscopy (TEM) during the course of life testing of <mml:math display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>∕</mml:mo><mml:mtext>air</mml:mtext></mml:mrow></mml:math>H2∕air polymer electrolyte fuel cells (PEFCs). In the fresh catalyst layers, anode Pt particles were found to have smaller particle sizes, better dispersion, and less agglomeration on the carbon-support surfaces than did the cathode <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>Pt</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:mi>Cr</mml:mi></mml:mrow></mml:math>Pt3Cr alloy particles. The operation-induced agglomeration of catalyst particles was evaluated for both the anode and cathode after defined life testing periods. Agglomeration of catalyst particles occurred primarily during the first 500 h of testing, which was confirmed by both TEM analysis and electrocatalytic surface area measurement. After 500 h, degradation of the recast Nafion ionomer network within the catalyst layers likely contributes more significantly to MEA performance degradation. Migration of metal catalyst particles toward the interface between the catalyst layer and membrane was observed at both electrodes. The Pt anode catalyst was less stable than the <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>Pt</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:mi>Cr</mml:mi></mml:mrow></mml:math>Pt3Cr cathode catalyst under high current density and high humidity conditions, which was confirmed by the higher extent of migration observed for the pure Pt than for the <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>Pt</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:mi>Cr</mml:mi></mml:mrow></mml:math>Pt3Cr . Some Pt particles (from both electrodes) were found to migrate into the membrane during the testing period.