Palladium(0) nanoparticles were obtained in high densities from a novel type reverse micelle (or W/O microemulsion) of bis(N-octylethylenediamine)palladium(II) chloride (=[Pd(octen)(2)]Cl-2)/water/chloroform (= CHCl3). The shape of the nanoparticles was largely affected by the phase of the additive, sodium boron hydride (= NaBH4) reductant, and by the replacement of a small amount of water with methanol.