Metal　catalysts　play　an　important　role　in　industrial　redox　reactions.　Although　extensively　studied,　the　state　of　these　catalysts　under　operating　conditions　is　largely　unknown,　and　assignments　of　active　sites　remain　speculative.　Herein,　an　operando　transmission　electron　microscopy　study　is　presented,　which　interrelates　the　structural　dynamics　of　redox　metal　catalysts　to　their　activity.　Using　hydrogen　oxidation　on　copper　as　an　elementary　redox　reaction,　it　is　revealed　how　the　interaction　between　metal　and　the　surrounding　gas　phase　induces　complex　structural　transformations　and　drives　the　system　from　a　thermodynamic　equilibrium　toward　a　state　controlled　by　the　chemical　dynamics.　Direct　imaging　combined　with　the　simultaneous　detection　of　catalytic　activity　provides　unparalleled　structure–activity　insights　that　identify　distinct　mechanisms　for　water　formation　and　reveal　the　means　by　which　the　system　self-adjusts　to　changes　of　the　gas-phase　chemical　potential.　Density　functional　theory　calculations　show　that　surface　phase　transitions　are　driven　by　chemical　dynamics　even　when　the　system　is　far　from　a　thermodynamic　phase　boundary.　In　a　bottom-up　approach,　the　dynamic　behavior　observed　here　for　an　elementary　reaction　is　finally　extended　to　more　relevant　redox　reactions　and　other　metal　catalysts,　which　underlines　the　importance　of　chemical　dynamics　for　the　formation　and　constant　re-generation　of　transient　active　sites　during　catalysis.　©　2021　The　Authors.　Advanced　Materials　published　by　Wiley-VCH　GmbH