Two basic experiments for the measurement of local (electro-)chemical potentials in crystalline mixed conductors, which are exposed to external forces, are reported. (a) The profile of the chemical potential of silver metal in a Wagner-Hebb polarization cell AgAgBrPt is measured by means of local potential probes. Deviations from calculated profiles are discussed in terms of either a possible electronic overvoltage at the anodic interface AgBrPt or a change of the defect structure of AgBr in the region near the anode. (b) In a second experiment, the ionic overvoltage at an interface AgBrAgCl is determined, again by means of local potential probes.