This study investigates the high-temperature stability and phase composition of two high-entropy oxides (HEOs), (Mn0.2Co0.2Ni0.2Cu0.2Fe0.2)Fe2O4 and (Mn0.2Co0.2Ni0.2Cu0.2Mg0.2)Fe2O4, prepared as single-phase samples using the reactive flash sintering technique. Results show that the annealing temperature in a nitrogen atmosphere has a significant impact on the stability of the compounds. The destabilization of the spinel structure occurs in a twostep process: spinel HEO -> spinel HEO + Fe2O3 -> spinel HEO + Cu based-oxide. This sequence is inferred from in-situ XRD experiments and calorimetric analysis, and confirmed by TEM observations. Impedance spectroscopy analysis revealed a complex, thermally activated electrical response comprising bulk and grain boundary contributions. AC conductivity follows Jonscher’s universal power law, with a temperature dependence of the S parameter consistent with overlapping large polaron tunneling. These findings provide insight into charge transport and relaxation processes in the prepared HEOs, improving their understanding for potential electrical applications.