Aluminas from various sources were thermally treated in air to obtain a range of phases (delta, theta, and alpha) with BET surface areas between 5 and 90 m(2)/g. Rhodium was impregnated onto the aluminas at a constant loading of 0.6 mu mol Rh/m(2) (BET). The resultant Rh-Al2O3 catalysts were hydrothermally aged by treating in 10% H2O/air at 1223 K for 24 h. Temperature-programmed reduction and a noble metal surface area measurement based on methanation of adsorbed CO were used to characterize the state of the Rh after aging. Three broad forms of Rh were identified : occluded, strongly interacted, and noninteracted. Apparent dispersions ranged from 3 to 11%, with the highest values obtained for alpha-alumina. A pronounced loading effect was observed for the Rh/alpha-alumina catalysts; dispersions ca. 10% were obtained only on samples with loadings less than about 1 mu mol Rh/m(2) (BET). Higher loadings (>1.5 mu mol Rh/m(2) (BET)) resulted in apparent dispersions of 3% or less. These results suggest that the most effective way to maximize the Rh dispersion in high-temperature aged alumina-based automotive catalysts is to isolate the Rh on alpha-phase alumina at loadings less than 1 mu mol/m(2) (BET). Presteaming of selected aluminas at 1223 K for 24 h prior to Rh impregnation decreased the BET surface area and resulted in less occlusion of Rh during subsequent aging.