The electrocatalytic oxygen reduction reaction (ORR) is central to alternative energy systems and sensors. An important practical issue in these systems is the loss of active surface area of the catalyst. The unexpected dissolution of Pt at moderately negative potentials during ORR in water and organic media was detected by combining nanoelectrochemistry with AFM imaging. The possibility to connect this phenomenon with the previously observed formation of hydroxyl radicals has been explored. The loss of Pt occurred only under the experimental conditions at which the formation of hydroxyl radicals was reported in the literature. This process can contribute to cathode degradation in fuel cells and other electrochemical systems. In electrochemical experiments employing a Pt electrode at negative potentials, the reduction of oxygen may be accompanied by the dissolution of the electrode surface.