Ultrasmall metal nanoparticles (NPs) with a mean diameter below 2 nm are being intensively studied due to their unique structural and chemical properties. At such small sizes, metal particles can appear as various polyhedra with different atomic structures depending on whether electronic effects, surface energy, or the nucleation mechanism govern their crystallization. Therefore, the synthesis of monodisperse nanoparticles of a very small size and well-defined structure requires a good understanding of the different steps of the crystallization process, which can be achieved by conducting and coupling in situ studies at different length and time scales. In this article, we describe the synthesis of ultrasmall gold NPs by reduction of HAuCl₄ in solution of oleylamine (OY) in hexane using trialkylsilanes as reducing agents. Thanks to time-resolved in situ small-angle X-ray scattering and X-ray absorption spectroscopy kinetic studies, a competition between nucleation of Au NPs from a solution containing Au(III) clusters and crystallization of a lamellar phase of composition OY-Au(I)-Cl was revealed. In situ X-ray diffraction and pair distribution function (PDF) analysis showed that the first chemical pathway leads to icosahedral NPs while the reduction of OY-Au(I)-Cl leads to fcc NPs. Increasing the reaction rate, achieved by adjusting the silane concentration, changing the nature of the silane (triethylsilane instead of triisopropylsilane), and/or increasing the temperature of reaction, avoided the formation of the Au(I) lamellar phase as the intermediate, leading to monodisperse Au NPs with an icosahedral structure. This fairly simple liquid-phase synthesis method yields highly concentrated suspensions of icosahedral gold NPs, paving the way for their future use in practical applications such as catalysis.