Microstructural changes in alumina-supported rhodium catalystsduring aging in air at high temperatures were investigated usinga combination of high-resolution transmission electronmicroscopy, X-ray photoelectron spectroscopy (XPS), and X-raydiffraction. Both constituents, particles and support, undergostructural changes during thermal aging. With increasing agingtemperature and time, the support transforms from γ- toα-alumina, and the phase change is accelerated by thepresence of rhodium. The rhodium, present initially as10-Å-diameter metal particles, undergoes oxidation with theformation of a variety of oxide phases, depending on thetemperature at which aging has occurred. At 500°C, highlydispersed, raft-like structures are observed. Continued aging atthis temperature results in a coarsening of these structures toform three-dimensional particles of high-temperature,high-pressure (HT, HP) orthorhombic Rh2O3. At 850°C, aging causes severe coalescence of the HT, HP orthorhombic Rh2O3particles, while at 1000°C, RhO2particles are observed together with large particles of HT, HP orthorhombic Rh2O3. Ahigh Rh binding energy (near 309 eV) is observed using XPSfollowing all aging treatments and may be attributed to eitherthe HT, HP orthorhombic phase or to Rh4+.