This thesis discusses the effect of initial particle size and alloying of Pt nanocatalysts on the degradation of Proton Exchange Membrane Fuel Cells (PEMFC). Platinum nanocatalysts with initial particle sizes of 2.2nm, 3.2nm, 5.0nm, 6.7nm and 11.3nm were studied, before and after potential cycling. The two smallest initial particle sizes show significant degradation, while the remainder of the samples show negligible degradation after 10,000 cycles. Among the possible degradation mechanisms operating, the results show that dissolution and re-precipitation is insignificant among all the samples. On the other hand, modified electrochemical Ostwald ripening (MEOR) is the main cause for particle growth and degradation of the Pt nanocatalysts. Moreover, MEOR could also assist the coalescence of particles. Thus, controlling the Pt dissolution rate is the key factor to prevent degradation. In the case of Pt₃Co nanocatalysts, both MEOR as well as dissolution & reprecipitation play an irrelevant role in degradation. However, particle migration and coalescence seems to be more severe in Pt₃Co nanoparticles than for Pt nanoparticles. / text