An investigation has been made on the chemical stability of the cathode-interconnect interface in solid oxide fuel cells. Lanthanum calcium chromite and lanthanum strontium manganite (a dense 10% A-site deficient manganite, a porous 10% A-site deficient one or a dense 1% A-site deficient one) were placed between air and fuel (hydrogen/water) at a selected electrical current density. The electrical conductivity across the interfaces was slightly increased for 300 h. Changes in morphology, chemical composition, and phases were examined by scanning electron microscopy/energy dispersive x-ray and x-ray diffractometry analysis. The dense 10% A-site deficient cathode gave rise to the precipitation of manganese oxide at the air-side surface as well as at the interface. The porous cathode enhanced chemical reactions between lanthanum calcium chromite and lanthanum strontium manganite. The dense 1% A-site deficient cathode was most stable. These phenomena have been thermodynamically analyzed in terms of (i) irreversible mass transfer under an oxygen potential gradient, (ii) changes of the stable composition region of the perovskite phases as a function of oxygen potential, and (iii) an enhancing effect of the liquid formation on reactions at interfaces.