The IrO₂ top electrode chemistry of ferroelectric IrO₂/Hf_0.5Zr_0.5O₂ (HZO)/IrO₂ metal–insulator–metal (MIM) structures dependent on rapid thermal annealing in different gas atmospheres (nitrogen, oxygen, and forming gas [FG]) is investigated. Using hard X-ray photoelectron spectroscopy (HAXPES), the strongly modified chemical states of the IrO₂ layer dependent on the choice of annealing gas atmosphere are observed. For O₂ and N₂ anneals, the IrO₂ electrode remains either unaffected or is just slightly chemically attacked at the top. In contrast, FG annealing causes a complete reduction of the IrO₂ top electrode into metallic Ir. Surprisingly, oxygen is detected—unbound to Ir—which is incorporated in the metallic Ir layer. This mobile oxygen is thought to affect the electrical behavior of the IrO₂/HZO/IrO₂ device. In addition, it may serve as a test sample for future studies of the root causes of the role of oxygen-vacancy interactions at the interface, which can influence the performance instabilities in HZO-based MIM structures, such as wake-up, imprint, and fatigue.