<p>This study investigates the high-temperature oxidation behaviour of the refractory high-entropy alloy (rHEA) Hf_0.5Nb_0.5Ta_0.5Ti_1.5Zr in air. The oxidation kinetics and thermal effects were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). As temperature and exposure duration increased, the initial stage of oxidation showed a roughly linear mass gain before changing to nonlinear behaviour. The oxidation rate gradually decreased at higher temperatures, eventually showing parabolic behaviour. TaO₅, ZrO₅, NbO₅, and other mixed oxides were formed as a result of oxygen diffusion into the alloy. Cracking, pore formation, and oxide scale spallation indicated severe degradation near a critical temperature threshold. The misalignment of the alloy’s thermal expansion coefficients with the complex oxides that develop while exposure to high temperatures is the cause of this disastrous oxidation.</p> Graphical abstract <p></p>