A comprehensive study of the mechanism of gold nanoparticle formation has been conducted using third-generation synchrotrons. The particles were obained by reduction of AuCl3 by BH4 in toluene. Gold oxidation state was monitored by X-ray absorption near edge spectroscopy (XANES), while the size and concentration of the nanoparticles were assessed by small-angle X-ray scattering (SAXS). A time-resolution of 100 ms has been achieved for a total formation time of a few seconds. The change with time of the total amount of Au(0) present in the solution is obtained by XANES. The change of the amount of Au(0) inside the nanoparticles is obtained from the SAXS signal. The comparison between these two quantities shows that a measurable amount of Au(0) exists transiently as monomers (or very small entities) in solution and this quantity is linked to an observed burst of nucleation of nanoparticles. The reduction kinetics is strongly influenced by the presence of ligands and a change in temperature. A model coupling the observed reduction kinetics and nucleation and growth laws is able to recover the final size and number densities of the explored experimental conditions.