The low-temperature alloying of Ag/Pd nanoparticles synthesized in a fatty acid film by a novel ion-entrapment process is described. Nanoparticles of silver and palladium were grown in thermally evaporated fatty acid (stearic acid) films by immersion of the film sequentially in solutions containing Ag+ ions and Pd2+ ions followed by their in-situ reduction at each stage. Incorporation of Ag+ and Pd2+ ions into the stearic acid film occurs by selective electrostatic binding with the carboxylate ions in the fatty acid matrix. It is observed that the reduction of the Pd2+ ions in the stearic acid-Ag nanocomposite film leads to the formation of a mixture of individual Ag and Pd nanoparticles as well as particles in the Ag-core/Pd-shell structure. Thermal treatment of the stearic acid-(silver + palladium) nanocomposite film at 100 °C resulted in the formation of an Ag-Pd alloy. The process of Ag+ and Pd2+ ion incorporation in the stearic acid matrix, their reduction to form metallic nanoparticles, and synthesis of the Ag-Pd alloy were followed by quartz crystal microgravimetry (QCM), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), UV-vis spectroscopy, and X-ray diffraction (XRD) measurements.