The low-temperature alloying of bimetallic Au-Pt nanoparticles entrapped within thermally evaporated fatty amine films by a novel ion-exchange technique is described. Nanoparticles of gold and platinum were grown in thermally evaporated octadecylamine films by immersion of the film sequentially in solutions containing AuCl4- ions and PtCl62- ions followed by their in situ reduction at each stage. Entrapment of AuCl4- and PtCl62- ions in the octadecylamine (ODA) film occurs by selective electrostatic binding with the ammonium ions in the fatty amine matrix. This process leads to the formation of a Au-core/Pt-shell structure of the nanoparticles within the lipid matrix. Thermal treatment of the ODA-(Au-core/Pt-shell) nanocomposite film at 150 °C resulted in the formation of a Au-Pt alloy. The process of AuCl4- and PtCl62- ion incorporation in the ODA matrix and synthesis of the Au-Pt alloy were followed by quartz crystal microgravimetry, Fourier transform infrared spectroscopy, UV-vis spectroscopy, and X-ray diffraction measurements.