Rotating disk electrode experiments were used to demonstrate the influence of dissolved Ru species on the oxygen reduction activity of a Pt/ C electrocatalyst. Dissolved Ru in micromolar levels was found to deposit instantly onto Pt, thereby blocking the electrode surface for ORR at low overpotentials. Ru contamination can decrease oxygen reduction kinetics by eightfold or increase the overpotential by ca. 160 mV. This facet of fuel cell durability needs special attention from the perspective of appropriate materials choice, i. e., preventing the leaching of Ru from PtRu anodes and its crossover to the cathode across the membrane electrolyte. (c) 2007 The Electrochemical Society.