The low temperature alloying of copper and nickel nanoparticles synthesized in a fatty acid film by a novel ion-entrapment process is described. Nanoparticles of copper and nickel were grown in thermally evaporated stearic acid films by immersion of the film sequentially in solutions containing Cu2+ ions and Ni2+ ions followed by their in-situ reduction at each stage. Entrapment of Cu2+ and Ni2+ ions in the stearic acid film occurs by selective electrostatic binding with carboxylate ions in the fatty acid matrix. Thermal treatment of the stearic acid-(Cu + Ni) nanocomposite film at 100 °C resulted in the formation of a Cu-Ni alloy. The process of Cu2+ and Ni2+ ion incorporation in the stearic acid matrix and synthesis of the Cu-Ni alloy were followed by quartz crystal microgravimetry (QCM), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD).