The chemical reactivity of perovskite oxide SOFC cathodes and yttria stabilized zirconia (YSZ) solid electrolyte was studied. The perovskite oxides that were examined were La1-xSrxCo0.2Mn0.8O3-d, La1-xSrxCo0.2Fe0.8O3-d and La1-xCaxCo0.2Fe0.8O3-d (0 less than or equal to x less than or equal to 0.5), as well as, the A-site deficient oxides La0.6-zSr0.4Co0.2Fe0.8O3-d, La0.6Sr0.4-zCo0.2Fe0.8O3-d and (La0.6Sr0.4)(1-z)Co0.2Fe0.8O3-d (0 less than or equal to z less than or equal to 0.2). Equimolar perovskite/YSZ powder mixtures were prepared and annealed at 1100 degreesC for 120 h. X-ray diffraction analysis was conducted to identify any reaction products. Si-standard was used as an internal standard for d-value calibration, during the determination of the lattice parameter of cubic YSZ. The main reaction products were La2Zr2O7 for the undoped and Lightly doped Fe-containing compositions and (Sr/Ca)ZrO3 for the Sr/Ca doped compositions. Also, CoFe2O4 was formed in the case of the Fe-containing compositions and monoclinic zirconia for La1-xCaxCo0.2Fe0.8O3-d with x = 0.4, 0.5. A-site deficiency in La0.6Sr0.4Co0.2Fe0.8O3-d-based perovskites reduced the amount of SrZrO3 formed, only in the case of La0.6Sr0.4-zCo0.2Fe0.8O3-d with 0 less than or equal to z less than or equal to 0.2 and (La0.6Sr0.4)(1-z)Co0.2Fe0.8O3-d with z=0.2. The lattice parameter of YSZ exhibited a shift, corresponding to a lattice contraction, due to the diffusion of the transition metal cations from the perovskite into YSZ. (C) 2000 Elsevier Science B.V. All rights reserved.