We studied the phase diagrams of microemulsions with a view to using these systems for the synthesis of metallic Pt, Pb, and Bi nanoparticles as well as of intermetallic Pt/Pb and Pt/Bi nanoparticles. The microemulsions consisted of H2O/salt-n-decane-SDS-1-butanol. The salt was either one metal precursor (H2PtCl6 center dot 6H(2)O, Pb(NO3)(2), or Bi(NO3)(3)center dot 5H(2)O), a Mixture of two metal precursors (H2PtCl6 center dot 6H(2)O + Pb(NO3)(2) or H2PtCl6 center dot 6H(2)O + Bi(NO3)(3)center dot 5H(2)O), or the reducing agent (NaBH4). In addition, other salts needed to be added in order to solubilize the metal precursors, to stabilize the reducing agent, and to adjust the ionic strength. Combining the microemulsion (mu e1) that contains the metal precursor(s) with the microemulsion (mu e2) that contains the reducing agent leads to metallic nanoparticles. To study systematically how the shape and size of the synthesized metallic nanoparticles depend on the size and shape of the respective microemulsion droplets, first of all one has to find those conditions under which mu e1 and mu e2 have the same structure. For that purpose we determined the water emulsification failure boundary (wefb) of each microemulsion as it is at the wefb where the water droplets are known to be spherical. We found that the ionic strength (1) of the aqueous phase as well as the hard acid and hard base properties of the ions are the key tuning parameters for the location of the wefb. (C) 2008 Elsevier Inc. All rights reserved.