<p>The Mo-40wt.%Cu pseudo-alloy, valued for its thermal conductivity and electrical performance, is used in heat sinks for high-density integrated circuits and power semiconductors, as ultrahigh-voltage electric contact materials in heavy-duty switches, circuit breakers, and electrodes, as sealing materials in microwave packages and as substrates for radar transistors and MEMS sensors. The oxidation behavior of the Mo-40wt.%Cu alloy was investigated at 450–550 °C in air, revealing a two-stage process comprising initial catastrophic oxidation caused by the formation of a liquid phase in the oxide scale, followed by self-passivation. Thermogravimetric analysis demonstrated parabolic kinetics during the catastrophic stage, governed by diffusion-controlled mechanisms, with a parabolic rate constant of 2.6·10⁻⁵ kg²·m⁻⁴·s⁻¹ at 520 °C — three orders of magnitude higher than that of pure copper oxidation (2.5·10⁻⁸ kg²·m⁻⁴·s⁻¹ at 500 °C). Subsequent passivation led to a marked reduction in oxidation rate, culminating in a mass-gain plateau. X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy revealed the formation of a two-layered oxide scale: an inner layer of copper molybdates and an outer dense CuO layer when passivation begins. This CuO layer, upon achieving continuity and exhibiting low ionic conductivity, impedes both copper cation and oxygen anion diffusion, thereby stopping further oxidation. Unlike MoO₃‑deposit‑induced catastrophic oxidation of copper, this self-passivation mechanism in the Mo-40wt.%Cu alloy offers insights for designing protection technologies for pseudo-alloys against catastrophic oxidation.</p>