Fuel cells operated under cyclic conditions in automotive operations are more vulnerable to membrane and electrode material 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. Potential cycling between 0 and 1.2 V (RHE) for 500 cycles caused about 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 due to potential cycling 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. copyright The Electrochemical Society.