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Metal dissolution and redeposition are considered to be the most important degradation mechanism for Pt-based fuel cell electrocatalysts. Understanding key factors mitigating the dissolution and migration under realistic proton-exchange-membrane (PEM) fuel cells is crucial for improving their performance and durability. Using ordered and disordered PtCo electrocatalysts, we address how structural ordering and particle size can affect the dissolution of Co/Pt and their redepositioninto the membrane upon catalyst accelerated durability test in PEM fuel cells by statistical scanning transmission electron microscopy (STEM) and spectroscopic analysis. Consistent with the improved performance and durability, we observe that both Co and Pt dissolution were mitigated in the ordered PtCo catalyst compared to the disordered one. The suppressed Pt dissolution was evidenced from the relieved particle coarsening and significantly suppressed Pt redeposition/migration in the membrane after the durability test. Moreover, we reveal an optimum particle size range between 2–5 nm for ordered PtCo catalysts, which favors the highest structural ordering degree and hence the highest retention of Co. These results provide arationale for implementing ordered Pt intermetallic electrocatalysts in PEM fuel cells and further particle size optimization for improved durability. [ABSTRACT FROM AUTHOR]