Electrochemical reduction was performed on both polycrystals of calcia-stabilized zirconia (CSY) and single crystals of yttria-stabilized zirconia (YSZ) at 500 degrees C. A glass-encapsulation was used to prevent access of molecular oxygen from the gas phase and to realize a virtually two-dimensional geometry for the reacting crystal. The resulting blackening process which occurs by the advancement of a morphologically unstable reaction front was observed in-situ in a heating-stage by the use of an optical microscope.