The reaction at the interface of a solid electrolyte cell between air electrode (La0.5Ca0.5MnO3) and YSZ-electrolyte with different Al2O3-contents was investigated by electron microscopy (SEM) and x-ray diffraction (XRD). Formation of MnAl2O4 was detected in a 5 µm diffusion zone within the electrolyte. MnAl2O4 formation can be explained by diffusion of Mn-ions into the electrolyte and subsequent reaction with the α-Al2O3 grains during sintering. Cell performance and long-term stability in SOFC operation are not negatively affected by MnAl2O4 formation. However a rise in electrode resistance and slow delamination of perovskite oxide electrode were observed after some hours of electrolysis. This reaction is the consequence of oxygen gas pressure at the electrolyte in the MnAl2O4 diffusion zone. It is caused by local increase of electronic conductivity by MnAl2O4 formation. Long-term stability also for electrolysis conditions has been achieved by an additional intermediate YSZ-layer between air electrode and electrolyte.