The function of systems and devices in many technologically important applications depends on dynamic processes in complex environments not accessible by structure and property characterization tools. Fuel cells represent an example in which interactions occur under extreme conditions: high pressure, high temperature, in reactive gas environments. Here, scanning surface potential microscopy is used to quantify local potential at electrode/electrolyte interfaces in operating solid oxide fuel cells at 600 °C. Two types of fuel cells are compared to demonstrate two mechanisms of ionic transport at interfaces. Lanthanum strontium ferrite-yttria-stabilized zirconia (LSF-YSZ) and lanthanum strontium manganite-yttria-stabilized zirconia (LSM-YSZ) cross-sectional electrode assemblies were measured to compare mixed ionic electronic conducting and electronic conducting mechanisms. Direct observation of the active zones in these devices yields characteristic length scales and estimates of activation barrier changes.