Ruthenium nanocrystals were prepared upon decomposition of Ru 3 (CO) 12 at high temperature in the presence of alkylamine stabilizers. The reaction produced a dark brown colloid that was processed with toluene/ethanol as solvent/nonsolvent pair. The materials were characterized using high resolution electron microscopy and powder X-ray diffraction. The Ru nanocrystals were single crystals with a hcp structure, and displayed regular facets. At lower temperatures nucleation was followed by attachment to yield mesoscale polycrystalline branched ruthenium structures.