The effect of organic stabilizers on the structure of supported Pt-Cu nanoparticles was investigated for liquid-phase reduction methods. The crystallite size and copper content of Pt-Cu alloy nanoparticles decreased as the number of carboxylate groups in an employed stabilizer molecule increased (acetate [ succinate [ citrate). The steric and inductive effects of carboxylate groups suppress the reduction and crystal growth of platinum to increase the chance for copper to make alloy with platinum, but at the same time they destabilize metallic copper resulting in a segregation of copper as oxides. In a radiolytic synthesis method, the size and Cu content in Pt-Cu nanoparticles were similarly reduced in the presence of citrate, but the copresent reduction enhancer also had a strong influence. The final particle structure was controlled by the kinetics of reduction of metal precursors and the thermodynamics of metal-stabilizer complexes.