Metal nanoparticles (NPs) are used to catalyze the formation of molecular multielectron transfer products, for example, H 2, from single-electron reductants, such as radicals. Noble metals, like silver and gold, were very instrumental in unraveling the mechanism of this interfacial process. In this study, we explore the effect of the support, silica nanoparticles, on the catalytic production of H2 on silver from radiolytically produced C(CH 3) 2OH radicals and water. We obtain very high concentrations of stable silica-supported silver nanoparticles that remain suspended in solution for long periods of time. The presence of metallic silver particles on the silica surface further induces a very effective deposition of silver particles on the same silica particle leading to cooperative deposition of the silver. The silica support changes appreciably the reactivity of the silver NPs, reducing the yield of the molecular hydrogen produced at the high concentrations of the supported NPs to that of the primary molecular hydrogen G(H 2) = 0.45 molecules/100 eV from water radiolysis indicating that H 2 production at the surface of the silver is inhibited. A possible explanation is that the catalyst (Ag on SiO 2) catalyzes the disproportionation of the reducing radicals or the reduction of acetone at the expense of the H 2 evolution catalysis.