The structure of a catalyst often changes as a result of changes in the reactive environment during operation. Examples include changes in bulk phases, extended-surface structures, and nanoparticle morphologies; now we report real-time characterization of changes in the structure of a working supported catalyst at the molecular level. Time-resolved extended X-ray absorption fine structure (EXAFS) data demonstrate the reversible interconversion of mononuclear iridium complexes and tetrairidium clusters inside zeolite Y cages, with the structure controlled by the C2H4/H-2 ratio during ethene hydrogenation at 353 K. The data demonstrate break-up of tetrairidium clusters into mononuclear complexes indicated by a decrease in the Ir-Ir coordination number in ethene-rich feed. When the feed composition was switched to first equimolar and then to a H-2-rich (C2H4/H-2 = 0.3) feed, the EXAFS spectra show the reformation of tetrairidium clusters as the Ir-Ir coordination number increased again. When the feed composition was cycled from ethene-rich to H-2-rich, the predominant species in the catalyst cycled accordingly. Evidence confirming the structural change is provided by IR spectra of iridium carboryls formed by probing of the catalyst with CO. The data are the first showing how to tune the structure of a solid catalyst at the molecular scale by choice of the reactant composition.