Nanostructured particles of amorphous carbon-activated palladium metallic clusters have been prepared (in situ) at room temperature by ultrasound irradiation of an organometallic precursor, tris-mu-[dibenzylideneacetone]dipalladium [(phi-CH=CH-CO-CH=CH-phi)(3)Pd-2] in mesitylene. Characterization by elemental analysis, transmission electron microscopy, differential scanning calorimetry, X-ray diffraction, X-ray photoelectron spectroscopy, and BET surface area measurements shows that the product powder consists of nanosize particles, agglomerated in clusters of approximately 800 Angstrom. Each particle is found to have a metallic core, covered by a carbonic shell that plays an important role in the stability of the nanoparticles. The catalytic activity in a Heck reaction, in the absence of phosphine ligands, has been demonstrated.