Fuel cells operated under automotive cyclic conditions are more vulnerable to membrane and electrode degradation. Degradation of Pt catalyst due to potential cycling was characterized by loss of hydrogen adsorption (HAD) area and shift in the half-wave potential for oxygen reduction at a thin film catalyst rotating disk electrode. It is shown from an analysis of the assumptions involved in calculating the HAD area that uncertainty in the potential dependence of hydrogen coverage and the inability to separate a priori the double layer charging current at Pt can lead to a 34% underestimation of the HAD area of Pt. Potential cycling between 0 and 1.2 V (RHE) for 500 cycles caused about a 20-30% decrease in HAD area for two carbon-supported platinum catalysts. This decrease followed second order kinetics, indicating that the loss of surface area is probably caused by agglomeration of Pt particles due to carbon corrosion. Analysis of oxygen reduction kinetic losses shows an increase in Tafel slope probably due to a change in the morphology of the carbon support caused by corrosion reactions. Three mechanisms are discussed for the loss of surface area and activity of Pt catalyst due to cycling. (c) 2006 The Electrochemical Society.