Reactions between thick film LaMnO3 cathodes and YSZ substrates were investigated by ageing at 1450 degrees C. Also, subsolidus phase equilibria in the La2O3-Mn2O3-ZrO2 system were confirmed by X-ray powder diffraction analysis. A La2Zr2O7 phase formed on the YSZ/LaMnO3 interface. The Mn2O3 released in the reaction partly diffused in to YSZ and partly evaporated. Diffusion of Y and Zr into LaMnO3 was not detected. After a prolonged period of ageing (100 h) the cathode layer is separated along most of the YSZ/LaMnO3 interface with only a few sintered contacts. On the surface of large pores where LaMnO3 separated from the YSZ substrate "islands" of La silicate phase was found. Silica originated from grain boundaries in YSZ. The presence of La silicate phase could be a reason for separation of the LaMnO3 layer from YSZ substrate after prolonged high-temperature ageing.