Alloying metals to form intermetallics has been proven effective in tuning the chemical properties of metal-based catalysts. However, intermetallic alloys can undergo structural and chemical transformations under reactive conditions, leading to changes in their catalytic function. Elucidating and understanding these transformations are crucial for establishing relevant structure-performance relationships and for the rational design of alloy-based catalysts. In this work, we used CuZn alloy nanoparticles (NPs) as a model material system and employed in situ transmission electron microscopy (TEM) to investigate the structural and chemical changes of CuZn NPs under H₂, O₂ and their mixture. Our results show how CuZn NPs undergo sequential transformations in the gas mixture at elevated temperatures, starting with gradual leaching and segregation of Zn, followed by oxidation at the NP surface. The remaining copper at the core of particles can then engage in dynamic behavior, eventually freeing itself from the zinc oxide shell. The structural dynamics arises from an oscillatory phase transition between Cu and Cu₂O and is correlated with the catalytic water formation, as confirmed by in situ mass spectrometry (MS). Under pure H₂ or O₂ atmosphere, we observe different structural evolution pathways and final chemical states of CuZn NPs compared to those in the gas mixture. These results clearly demonstrate that the chemical state of alloy NPs can vary considerably under reactive redox atmospheres, particularly for those containing elements with distinct redox properties, necessitating the use of in situ or detailed ex situ characterizations to gain relevant insights into the states of intermetallic alloy-based catalysts and structure-activity relationships.