The electrocatalytic performance of nanoengineered PtNiFe catalysts in proton exchange membrane fuel cells (PEMFC) is described in this report. The membrane electrode assembly was prepared using carbon-supported PtNiFe nanoparticles treated at two different temperatures as the cathode electrocatalysts in PEMFC. The PtNiFe/C catalysts were found to exhibit excellent fuel cell performance, much better than that of commercial Pt/C catalyst. In addition to assessing the mass activities in the kinetic current region, the fuel cell performance was also determined in the ohmic and mass transport regions. The electrocatalytic and fuel cell performance are shown to depend on the thermal treatment temperature of the trimetallic catalysts. The higher-temperature treated catalysts showed a higher power density than the lower-temperature treated catalysts. The results are also discussed in terms of the effect of lattice shrinking in the trimetallic alloy nanoparticles on the electrocatalytic activity. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.