The effect of platinum loading on catalyst durability was investigated by subjecting two identical 50-cm(2) active area cells with 0.15 and 0.4 mg(Pt).cm(-2) in cathode to 30,000 triangle sweep cycles between 0.6 V and 1 V at 50 mV.s(-1) scan rate. The gas flow conditions were 80 degrees C, 1 atm, and 100% relative humidity for H-2 (fuel) and N-2 (oxidant). Both cells lost about 55% of the initial electrochemically active surface area and showed similar H-2 crossover rates and decreases in high frequency resistance. Analysis of the back-scan polarization data indicated that the Tafel slope increased by 5-10% over the initial 5,000 cycles, and then more gradually over the subsequent 25,000 cycles. Consistent with the XRD measurements that showed growth in the average size of Pt particles, the estimated exchange current density (mA.cm(Pt)(-2)) increased by similar to 20% over the test period for both cells. Both the activation overpotentials for oxygen reduction and the mass transfer overpotentials increased with cycling. The mass transfer overpotentials were higher in the 0.15-mg(Pt).cm(-2) cell and increased faster with cycling than the overpotentials in the higher Pt-loaded cell. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.jes113064] All rights reserved.