The Ni/(ZrO2 + 9.5 mol% Y2O3) interface was used as a model system to investigate decomposition reactions of a yttria-stabilized ZrO2 electrolyte in contact with a metal at elevated temperature. In the present study, the sample was a diffusion-bonded symmetrical galvanic cell NiZrO2 + 9.5 mol% Y2O3Ni. Various electron microscopy techniques were used to study the morphology and structure of the reaction products at the Ni–ZrO2 electrolyte phase boundary after current flow. Below a critical oxygen partial pressure of approximately 10−27 atm, an intermetallic reaction layer formed at the Ni–electrolyte interface. Between the intermetallic layer and electrolyte a thin Y2O3 layer was present, which acted as a diffusion barrier for Zr and Ni, slowing down the overall chemical reaction. At several locations at the interface the Y2O3 layer broke up, leading to a morphological instability of the interface between electrolyte and Ni5Zr, allowing further reaction. The thickness of the total reaction layer varied widely as a consequence of such an instability.