The effect of cation nonstoichiometry on the surface reactivity of mixed ionic and electronic conducting (La,Sr)CoO3 (LSC113)-based bi-layer thin film cathodes is investigated by ac impedance method as a function of thin film surface composition. The bi-layer thin film cathodes are fabricated by an RF-sputtering method. The results of X-ray diffraction and transmission electron microscopy observation prove that LSC thin films of A-site and B-site deficient composition crystallize in a perovskite structure, which is attained by retained supersaturated metastability from the fabrication process. Both A-site and B-site deficient LSC113 phases have surface reaction performance significantly higher than the cation-stoichiometric LSC113 phase, which is attributed to the increase of oxygen vacancy in charge-compensation for cation vacancy. The "cation nonstoichiometry" is one of high surface reaction models to improve solid oxide fuel cells' performance. (C) 2013 Elsevier B.V. All rights reserved.