(La1– xSrx)yCr0.5Mn0.45Ni0.05O3– δ (LSCMN) is a perovskite (ABO3) type mixed ionic-electronic conductive (MIEC) oxide and has been proposed as an electrode material for high temperature fuel cell [1]. The material is catalytically active for oxidation of hydrogen and hydrocarbons [2]. A significant amount of attention has been paid to clarify the dependence between crystallographic and electrical properties of LSCM-based materials [1, 3]. Some papers also deal with stability issues if the material is used as an anode in a solid oxide fuel cell [3, 4]. So far, there is a lack of information about the dependence between A-site stoichiometry and stability of chemical composition- and electrochemical performance of LSCMN surface.In this work, La/Sr ratio and deficiency of A-site, i.e. A-site stoichiometry of (La1– xSrx)yCr0.5Mn0.45 Ni0.05O3–δ were varied and chemical composition of LSCMN surface and electrochemical performance was monitored. The electrochemical measurements of symmetric cells (100 h tests) show that small stochiometric changes in A-site significantly influence the activity and initial degradation rate of the electrode. The chemical and structural changes of the material surface (segregation of some LSCMN components onto the surface and decomposition of perovskite phase) have a key role in the electrochemical performance of the electrode [3].The electrode materials were analysed using XRD, TOF SIMS and electrochemical methods. XRD results for studied electrode powders showed significant dependence on the lattice parameters on the A-site composition. Materials were treated in two different gas environments – in synthetic air and in H2 environment, both at 1123 K for one hundred hours. TOF SIMS analysis demonstrated the dependence of the surface stoichiometry on the A-site stoichiometry of bulk electrode as well as on gas environment during heat treatment. The results from impedance spectroscopy (measured at temperatures from 973 to 1123 K in H2 environment, at OCV) demonstrate a significant influence of the A-site deficiency and La/Sr ratio on the series- and polarization resistance of the electrode. The electrochemical activity and surface compositions were correlated and reasons of relative stoichiometric changes of the surface compared with bulk were discussed. Yue and J.T.S. Irvine, Solid State Ionics, 225, 131 (2012).Bidrawn, G. Kim, G. Corre, J.T.S. Irvine, J.M. Vohs, R.J. Gorte, Electrochem. Solid-State Lett. 11, B167 (2008).Korjus, J. Aruväli, P. Paiste, K.Kooser, S. Granroth, H. Mändar, M. Kodu, R. Härmas, M. Maide, M. Ainsar, E. Lust, G. Nurk, Fuel Cell, 20, 6, 741 (2020).C. Lu, J. H. Zhu, Solid State Ionics, 178, 1467 (2007).