This paper represents the study of oxidation processes in multicomponent high-entropy equiatomic FeCoNiMnCr alloy, which is crystallized in f.c.c. lattice. Multicomponent equiatomic FeCoNiMnCr alloy is fabricated by vacuum-arc melting. It is studied by in situ method in high-temperature attachment UVD-2000 at temperatures 293—1273 K using DRON UM1 diffractometer in the air. Crystalline structure of multicomponent cast of high-entropy equiatomic FeCoNiMnCr alloy in the initial state at the temperature of 293 K is determined by X-ray diffraction analysis as a single-phase solid solution with f.c.c. lattice ($a$ = 0.3609 nm). At the temperature of 873 K, the presence of MnO and Fe$_{2}$MnO$_{4}$ oxides is registered. Most likely, the formation of MnO oxide can be explained by the large chemical affinity of Mn to oxygen in comparison with other elements. Within the temperature range 293—773 K, coefficient of thermal expansion for the initial f.c.c. solid solution is $(13,9 \pm 0,2)\cdot10^{-6}$ K$^{-1}$. At the temperature of 1273 K, lattice period of f.c.c. solid solution is decreased slightly ($a$ = 0.3596 nm). Obviously, this can be explained by the depletion of the surface of the solid solution by Mn atoms with the largest atomic radius amongst other metallic elements in the formation of the surface of the oxide. Compared to the initial state of the sample, the lattice period of f.c.c. solid solution is decreased, because of the redistribution of atoms in the crystal lattice and the decreased internal stresses. Microhardness of high-entropy FeCoNiMnCr alloy after oxidation is 2.1 $\pm$ 0.2 GPa, and the Young’s modulus is 130 \pm 5 GPa.