In this article, we provide a detailed description of the synthesis and properties of Pt dendrimerencapsulated nanoparticles (DENs) prepared using sixth-generation, hydroxyl-terminated, poly(amidoamine) (PAMAM) dendrimers (G6-OH) and three different PtCl42-/G6-OH ratios: 55, 147, and 240. Results obtained from UV-vis spectroscopy, X-ray photoelectron spectroscopy, electron microscopy, X-ray absorption spectroscopy, and high-energy X-ray diffraction show that only a fraction of the Pt 2+/dendrimer precursors are reduced by BH4- and that the reduction process is highly heterogeneous. That is, after reduction each Pt2+/dendrimer precursor complex is either fully reduced, to yield a DEN having a size and structure consistent with the original PtCl 42-/dendrimer ratio used for the synthesis, or the precursor is not reduced at all. This result is consistent with an autocatalytic process that entails slow formation of a nascent catalytic Pt seed within the dendrimer, followed by rapid, catalytic reduction of nearby Pt2+ ions. Details concerning the formation of the Pt2+/dendrimer precursor are also discussed.