Improved anode CO tolerance is a promising approach for integrating low-temperature PEM fuel cells with hydrocarbon fuel processors in cost-effective systems for portable and stationary power applications. PtSn@Pt core-shell nanoparticle electrocatalysts - created by applying cyclic potentials in the presence of CO to PtSn intermetallic nanoparticles in rotating disk electrode (RDE) experiments - have demonstrated the potential for high CO tolerance at low temperatures. This study explores the use of potential cycling with full PEM fuel cell membrane electrode assemblies (MEAs), initially with PtSn anode electrocatalysts, to produce PtSn@Pt electrocatalysts in situ for increased anode CO tolerance. Potential cycling of PtSn anodes in MEAs with various gaseous feeds consistently showed less dramatic decreases in CO oxidation overpotentials than observed in RDE studies. Although some results suggested that modified PtSn electrocatalysts outperform state-of-the-art PtRu anode electrocatalysts, PtSn@Pt electrocatalysts formed via MEA potential cycling consistently did not provide adequately low anode overpotentials with CO up to 1000 ppm to outperform commercial PtRu anode catalysts. Energy-dispersive X-ray spectroscopy of MEA cross-sections showed that Sn leached from the anode into the cathode as the number of cycles increased. Consistent formation of PtSn@Pt core-shell structures for high CO tolerance in full MEAs remains a challenge for further investigation.