The synthesis of colloidal Pt particles obtained from the reaction between platinum acetylacetonate (Pt(acac)(2)) and Al(CH3)(3) was monitored, and the formation of the Pt colloid via "reductive stabilization" was established by a combination of techniques including NMR, anomalous small-angle X-ray scattering (ASAXS), X-ray absorption spectroscopy (XAS), density functional theory (DFT) computations, and transmission electron microscopy (TEM). For the first time, a defined organometallic compound was identified as the common intermediate in the formation of an organometallic Pt complex or a Pt colloid, depending on the type of ligands or stabilizers present. During the. decomposition of this intermediate state, stable colloidal particles of just one size are observed the number of which increases with time.