In order to provide insight into the interfacial relationships at fuel cell electrodes, a measuring technique based on a scanning tunneling microscope working in an electrochemical cell has been developed. The structure of model electrodes consisting of carbon supported Pt and ionomer mixtures as well as MEA electrode surfaces is imaged by the electrochemical scanning tunneling microscope (EC-STM) from microscale to nanoscale. Images with subnanometer resolution are obtained indicating that some ordering of the particles on the carbon support is present in the model system and in the MEA. It is demonstrated that in both cases nanometer structures can be imaged reliably and that the interface is rather clean and active under reaction conditions. In addition, a technique has been developed to measure the local reactivity by using the STM tip as a sensor electrode for the ORR.