Thermodynamic calculations on high temperature reactions between an YSZ electrolyte and LaCoO3 cathode materials identify the limiting conditions of temperature and oxygen partial pressure ranges required to prevent formation of resistive reaction products. The time dependence of the fraction reacted was evaluated under isothermal conditions and as a function of temperature. Theoretical models for both diffusion or phase boundary controlled reactions fail to fit the experimental data. This failure is probably related to loss of contact at the LaCoO3\La2Zr2O7 interface, when significant oxygen pressures are built up inside compact reacting mixtures of YSZ and LaCoO3.