A simple one-step and gram-scale synthesis of [Au5Mes5] from AuCl3 was developed, and this molecular precursor was used to generate Au nanoparticles on SiO2 and Al2O3. While [Au5Mes5] does not react with surface silanols and is only physisorbed, its incipient wetness impregnation followed by H2 treatment leads to a narrow size distribution of 1.4 nm Au nanoparticles. In contrast, [Au5Mes5] reacts with partially dehydroxylated Al2O3 to directly yield 1 nm Au nanoparticles along with adsorbed species. A subsequent treatment under hydrogen leads to a narrow size distribution of smaller 0.8 nm Au particles.