The reduction of bis(acetylacetonato)platinum(II), nickel(II) acetate tetrahydrate and molybdenum(II) acetate dimer in a mixed solvent of oleylamine and diphenyl ether using 1,2-hexadecanediol resulted in the production of oleylamine-protected ternary alloy nanoparticles (nickel-molybdenum-platinum ternary alloy nanoparticles (NiMoPtNPs)). A ligand-exchange reaction with 50% N-methylated linear poly(ethyleneimine-co-N-methylethyleneimine) (poly(EI-co-NMEI)) in chloroform yielded poly(EI-co-NMEI)-protected NiMoPtNPs. The resultant nanoparticles had average diameters of 1.9-2.5 nm and could be dispersed in water at pH levels ranging from -1 to 14. The average radii of the NiMoPtNPs decreased when the protecting polymer was changed from oleylamine to poly(EI-co-NMEI). Energy-dispersive X-ray spectroscopy of the poly(EI-co-NMEI)-protected NiMoPtNPs was performed to confirm the formation of polymer-protected alloy nanoparticles of nickel, molybdenum and platinum.