Herein, electroreduction in yttria-stabilized zirconia are investigated by means of Hebb-Wagner polarization experiments. By performing optical and thermal microscopy on single crystals and thin films during the application of an electric field under vacuum or oxygen-tight sealed conditions, the movement of the reduction front from the cathode to the anode, which causes a blackening of the material, is monitored. When performing electrocoloration experiments on thin film samples, the progressing reaction of the blackened region was found to be inhomogeneous and evolves as a dendrite-like finger structure. The progression of the blackening fingers follow preferentially the electric field lines and thus are influenced by distortions in the field that can be caused by metallic particles embedded in the oxide. In contrast to this, in the first stage of the reduction process no significant influence of mechanically-induced dislocations on the morphology or kinetics on the electroreduction can be found. Only after a heavy electroreduction was a localized transformation of the surface region observed. There is an evolution of highly oxygen deficient ZrO_{x} regions, which have a characteristic checked topography pattern at the microscale level.