This study treats the synthesis of silica-supported iridium metal particles by several different methods. Iridium trisacetylacetonate was first deposited on silica either by sublimation or by impregnation from a toluene solution. Infrared study showed no difference between the methods, each of which produced Ir(acac)(3) physisorbed at the surface, The physisorbed precursor was transformed by two methods and the reactions were followed by in situ infrared spectroscopy. In the first method, the solid was first heated under a flow of oxygen to produce surface iridium oxide, which was then reduced under hydrogen at different temperatures to provide iridium metal support particles. In the second method a physisorbed precursor was directly reduced under a hydrogen flow. Electron microscopy showed that both methods produced narrow distributions of metallic particle sizes between 1 and 5 mm, but for the first method very large metallic aggregates were also observed. The chemisorption of hydrogen, oxygen, and CO on the resultant supported metal materials at 25 degrees C was investigated as a means of determining the dispersion of the samples. A discrepancy between the dispersions deduced from chemisorption of H-2, O-2, and CO and from electron microscopy on the resultant supported metal materials was tentatively interpreted as an indication that some very small Ir particles, present on the silica surface, were not detected.