PtPb intermetallic nanoparticles (3–6 nm) with precise control (±0.8 nm) were prepared under mild conditions by using water-in-oil (w/o) microemulsions as reaction media. The amount of the aqueous phase has no significant effect on the composition of the NPs, while it indeed affects the size of the resulting NPs in a systematic way. A precise size control can be obtained by controlling the size of the w/o-microemulsions containing the metal salts and the reducing agent, respectively.