Microstructural changes in alumina-supported rhodium catalysts during aging in air at high temperatures were investigated using a combination of high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction. Both constituents, particles and support, undergo structural changes during thermal aging. With increasing aging temperature and time, the support transforms from gamma- to alpha-alumina, and the phase change is accelerated by the presence of rhodium. The rhodium, present initially as 10-Angstrom-diameter metal particles, undergoes oxidation with the formation of a variety of oxide phases, depending on the temperature at which aging has occurred. At 500 degrees C, highly dispersed, raft-like structures are observed. Continued aging at this temperature results in a coarsening of these structures to form three-dimensional particles of high-temperature, high-pressure (HT, HP) orthorhombic Rh2O3. At 850 degrees C, aging causes severe coalescence of the HT, HP orthorhombic Rh2O3 particles, while at 1000 degrees C, RhO2 particles are observed together with large particles of HT, HP orthorhombic Rh2O3. A high Rh binding energy (near 309 eV) is observed using XPS following all aging treatments and may be attributed to either the HT, HP orthorhombic phase or to Rh4+.