This article attempts to study changes in the microstructure of solid solutions with the perovskite structure La0.5 Ca0.5 Mn0.5 Fe0.5 O3 under the action of the methane oxidation reaction medium. By the methods of XRD, XPS and HRTEM the initial condition of the structure and the surface of the perovskite were both investigated. A feature of the structure of this solid solution is the presence of planar defects in the direction of the planes (101). After the methane oxidation reaction, a similar study of perovskite structure was conducted to obtain the changes. It was shown that under the action of the reaction medium, Ca1−x Mnx O particles form on the surface of the perovskite phase, while planar defects in La0.5 Ca0.5 Mn0.5 Fe0.5 O3 structure remain. In situ XRD experiments on perovskite calcination in helium current up to 750◦ C showed the formation of a similar Ca1−x Mnx O phase on the perovskite surface.