In many liquid-phase synthesis methods developed to produce nanomaterials, the key parameters governing the selective synthesis of solids and compounds are clearly identified, e.g. heat treatment profile, precursors solubility, pH, etc. Most of these well-understood approaches rely on relatively low temperature processes, below 400 °C, where conventional solvents are still stable. Interestingly, thermally stable inorganic molten salts enable to widen the temperature range for liquid-phase syntheses. They provide access to other families of crystalline solids requiring higher temperatures, as multicationic oxides. Nonetheless, the mechanisms that govern solid state formation and phase selection when different compounds compete are poorly understood. Herein, we report how experimental parameters, such as temperature, time, reaction medium composition and solvent oxo-basicity, enable to drive the synthesis mechanisms in molten salts towards nanoscaled multicationic oxides. We especially enlighten the phase-selective synthesis of pseudo-cubic perovskite LaNiO3 and layered Ruddlesden-Popper phases La2NiO4 and LaSrNiO4 at the nanoscale, by suggesting that the oxidation state of the metallic precursor plays a key role in the reaction pathway. This allows designing electrocatalysts for oxygen reduction reaction.