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Several hydroxyapatite (HAP) supports were prepared by precipitation under different pH values (8.5, 9.5, or 10.5), and rhodium was loaded onto the supports by impregnation followed by calcination at 500 °C. The pH value during the precipitation was found to play an important role in determining the catalytic activity of the resulting catalysts in N₂O decomposition. The Rh/HAP catalyst prepared under optimal conditions (pH 10.5) showed superior catalytic activity than Rh/Al₂O₃, Rh/TiO₂, and Rh/SiO₂, due to the rich basic sites of the HAP support and the dispersion of ultra small rhodium particles on HAP. A pretreatment of Rh/HAP in 4% H₂ at 400 °C can further improve the activity owing to the partial reduction of the rhodium species. The catalysts/supports were characterized by X-ray diffraction (XRD), N₂ adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), CO₂ temperature …