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　H-2,　O-2　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　H-2　or　O-2　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.