PEM fuel cell membrane electrode assemblies with Nafion electrolytes and commercial Pt-based cathodes were tested with Pt 0.8 Mo 0.2 alloy and MoO x @Pt core–shell anode electrocatalysts for CO tolerance and short-term stability to corroborate earlier thin-film RDE results. Polarization curves at 70 °C for the Pt 0.8 Mo 0.2 alloy in H 2 with 25–1000 ppm CO showed a significant increase in CO tolerance based on peak power densities in comparison to PtRu electrocatalysts. MoO x @Pt core–shell electrocatalysts, which showed extremely high activity for H 2 in 1000 ppm CO during RDE studies, performed relatively poorly in comparison to the Pt 0.8 Mo 0.2 and PtRu alloys for the same total catalyst loading on a per area basis in MEA testing. The discrepancy is attributed to residual stabilizer from the core–shell synthesis impacting catalyst-ionomer interfaces. Nonetheless, the MoO x @Pt electrochemical performance is superior on a per-gram-of-precious-metal basis to the Pt 0.8 Mo 0.2 electrocatalyst for CO concentrations below 100 ppm. Due to cross-membrane Mo migration, the stability of the Mo-containing anode electrocatalysts remains a challenge for developing stable enhanced CO tolerance for low-temperature PEM fuel cells.