Abstract: Perovskite-like complex oxide La0.5Ca0.5Mn0.5Co0.5O3±δ synthesized by the Pechini method is characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Structural transformations of this compound in the media with different partial pressures of oxygen and in the medium with hydrogen are studied in situ for the first time. The solid solution in the medium with low partial pressure of oxygen is shown to be unstable. The structural state of the sample is affected not only by the presence/absence of oxygen in the medium but also by the rate of sample cooling. X-ray diffraction and thermal analysis reveal that perovskite undergoes polymorphic transition from orthorhombic to cubic symmetry at 600 °C due to oxygen loss. Reducing the sample by hydrogen leads to the formation of two perovskite-like phases with pseudo-cubic and orthorhombic symmetries, possibly, due to the inhomogeneous formation of vacancies in the solid solution and changes in the oxidation states of cobalt and manganese cations. It is shown that the initial structure can be partially regenerated from the complex oxide reduced in H2.