Noble metal clusters (Ag, An) were formed in a silica aerogel matrix by gamma-irradiation of hydrogel precursors loaded with aqueous solutions containing Ag+ or [AuCl4](-) ions. Hydrogels exposed to gamma-rays assumed the color expected for colloidal suspensions of Ag (respectively Au) clusters. The hydrogels were subsequently washed and supercritically dried, without any evident change in color, indicating that the metal clusters were not removed during drying. Typical gamma-ray doses were between 2 and 3.5 kGy, and achieved complete reduction of hydrogels containing ion concentrations in the 10(-4)-10(-3) M range. Reduction of solutions with higher metal ion concentrations was achieved by multiple irradiations. In the case of Ag' reduction, we noticed that the radiolytic yield G was higher than that of the reducing radicals equal to G = 6 x 10(-7) mol/L of reducing species per joule of absorbed energy. For example, a total dose of 3.5 kGy has led to a reduction of 5.7 x 10(-2) M in a hydrogel with [Ag+] = 0.1 M. The high radiolytic yield has been attributed to radiation-induced oxide surface catalysis, or to autocatalysis, but the precise mechanism has not been determined. Metal clusters in the aerogel monoliths were characterized with optical absorption, transmission electron microscopy, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. These techniques have shown that the clusters have a crystalline fcc structure. Au clusters consist of pure Au, while surface oxidation of Ag clusters was observed with XPS.