Very small ruthenium and platinum nanoparticles have been prepared from organometallic complexes by using 1,3,5-triaza-7-phosphaadamantane (PTA) as stabilizer, which allowed their easy dispersion into water. These nanoparticles were fully characterized by different techniques. In particular, the coordination of PTA to the surfaces of the particles was studied by NMR spectroscopy. The potential of these nanoparticles as catalysts was investigated in aqueous biphasic catalysis; olefins and arene derivatives were hydrogenated under mild conditions of temperature and pressure with interesting conversions and selectivities despite the change of environment they underwent after their dissolution in water.