Cobalt-nickel bimetallic nanoparticles were synthesized by changing the sequence of the chemical reduction of Co(II) and Ni(II) ions confined in the core of bis(2ethylhexyl)phosphate 2 , and Ni(DEHP) 2 . The reduction was carried out by mixing, sequentially or contemporaneously, fixed amounts of n-heptane solution of Co(DEHP) 2 and Ni(DEHP) 2 micelles with a solution of sodium borohydride in ethanol at a fixed (reductant)/(total metal) molar ratio. This procedure involves the rapid formation of surfactant-coated nanoparticles, indicated as Co/Ni (Co after Ni), Ni/Co (Ni after Co), and Co þ Ni (simultaneous), followed by their slow separation as nanostructures embedded in a sodium bis(2-ethylhexyl)phosphate matrix. The resulting composites, together with those obtained by reducing the n-heptane solutions of pure Co(DEHP) 2 or Ni(DEHP) 2 , were characterized by XPS, EXAFS, WAXS, and SAXS. The data analysis confirms the presence of nanometer-sized surfactant-coated cobalt, nickel, and cobalt/nickel particles. As expected, the composition and internal structure of cobalt/nickel bimetallic nanoparticles are influenced by the preparation sequence as well as by the "chemical affinity" between the surfactant and the metal. However, some atomic-scale physicochemical processes play a subtle role in determining the structural features of bimetallic nanoparticles. Further effects due to the competition between nanoparticle growing process and surfactant adsorption at the nanoparticle surface were observed.