Agglomerative sintering of an atomically dispersed, zeolite Y-supported catalyst, Ir-1/zeolite Y, formed initially from the well-characterized precatalyst [Ir(C2H4)(2)]/zeolite Y and in the presence of liquid-phase reactants, was monitored over three cycles of 3800 turnovers (TTOs) of cyclohexene hydrogenation at 72 degrees C. The catalyst evolved and sintered during each cycle, even at the relatively mild temperature of 72 degrees C in the presence of the cyclohexene plus H-2 reactants and cyclohexane solvent. Post each of the three cycles of catalysis, the resultant sintered catalyst was characterized by extended X-ray absorption fine structure spectroscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The results show that higher-nuclearity iridium species, Ir-n, are formed during each successive cycle. The progression from the starting mononuclear precursor, Ir-1, is first to Ir-similar to 46; then, on average, Ir-similar to 40; and finally, on average, Ir-similar to 70, the latter more accurately described as a bimodal dispersion of on-average Ir similar to 40-50 and on-average Ir-similar to 1600 nanoparticles. The size distribution and other data disprove Ostwald ripening during the initial and final stages of the observed catalyst sintering. Instead, the diameter-dispersion data plus quantitative fits to the cluster or nanoparticle diameter vs time data provide compelling evidence for the underlying, pseudoelementary steps of bimolecular agglomeration, B + B -> C, and autocatalytic agglomeration, B + C -> 1.5C, where B represents the smaller, formally Ir(0) nanoparticles, and C is the larger (more highly agglomerated) nanoparticles (and where the 1.5 coefficient in the autocatalytic agglomeration of B + C necessarily follows from the definition, in the bimolecular agglomeration step, that 1C contains the Ir from 2B). These two specific, balanced chemical reactions are of considerable significance in going beyond the present state-of-the-art, but word-only, mechanismthat is, actually and instead, just a collection of phenomenafor catalyst sintering of Particle Migration and Coalescence. The steps of bimolecular plus autocatalytic agglomeration provide two specific, balanced chemical equations useful for fitting sintering kinetics data, as is done herein, thereby quantitatively testing proposed sintering mechanisms. These two pseudoelementary reactions also define the specific words and concepts for sintering of bimolecular agglomeration and autocatalytic agglomeration. The results are also significant as the first quantitative investigation of the agglomeration and sintering of an initially atomically dispersed metal on a structurally well-defined (zeolite) support and in the presence of liquid reactants (cyclohexene substrate and cyclohexane solvent) plus H-2. A list of additional specific conclusions is provided in a summary section.