Ultrafine Pd nanoparticles with an extremely narrow distribution were prepared by sonochemical reduction of [Pd(NH3)(4)](2+) in the presence of I-propanol. The average diameter of the particles was approximately 5 nm. These nanoparticles were extremely stable in the colloidal state, even in the absence of stabilizers. After vacuum drying, the primary particles connected to each other and formed radially arranged nanostructures of about 30-50 nm.