[en] The synthesis of Fe_3O_4@Au nanoparticles has received much attention due to promising applications in the biomedical field. In this work, we produced Fe_3O_4@Au nanoparticles by using a two-step solvothermal route that employed Fe_3O_4 nanoparticles as seeds for the Au deposition. Although this protocol leads to highly monodisperse and reproducible Fe_3O_4@Au nanoparticles it was necessary to perform a systematic study to have a better understanding, improve the yield and allow us to obtain a tunable result. We demonstrated that the Au:Fe_3O_4 ratio is a key parameter that, contrary to what could be expected, does not influence the Au shell thickness. However, this parameter should be optimized because it strongly influences the yield. When the Au:Fe_3O_4 ratio was low there were plenty of uncoated Fe_3O_4 nanoparticles, whereas when the Au:Fe_3O_4 ratio was high there could be some pure Au nanoparticles together with the desired Fe_3O_4@Au nanoparticles. Furthermore we demonstrated that the Au shell thickness can be tuned by varying the reaction temperature. This paper describes the influence of both parameters and proposes a mechanism of the synthetic process by studying parametrically the morphological and structural evolution of the nanoparticles by TEM, DLS, SQUID and UV–vis spectroscopy. (paper)