Aqueous solutions of Ag-Pt ions and poly(vinyl alcohol) were irradiated with gamma rays at dose rates below 0.5 kGy/h to generate nanoparticles. The nanoparticles were characterized with several experimental techniques. Transmission electron microscopy showed that, surprisingly, the nanoparticles were not spherical but had a high aspect ratio. Wirelike structures were generated with lengths Lip to 3.5 mum and diameters between 3 and 20 nm. Selected-area diffraction showed that the wires were polycrystalline and that individual grains making up the wires had a face-centered cubic (fee) structure, The optical absorption of samples, with a Ag/Pt mote ratio higher than 80%, exhibited a Ag surface plasmon absorption band centered around 400 nm. The plasmon band broadened with increasing Pt molar ratio and was replaced by a monotonically decaying background for a Pt molar ratio higher than about 30%. Alloying in the Ag-Pt nanoparticles was investigated with X-ray absorption spectroscopy. The Pt L edge ( 11.564 keV) was excited to determine the local structure around the Pt atoms. A contraction in the first shell of 0.05 Angstrom was observed, which ruled out the formation of a Ag-Pt homogeneous alloy and suggested the formation of core-shell particles, To understand the mechanism of formation of the nanoparticles. several experimental parameters such as the total radiation dose, type of polymer, metal and polymer concentrations, and type of counterions in solution were varied. The most relevant parameters inducing filament growth were the counterions added to the solution, the mole ratio between the two metals, and the capping polymer. For example, spherical particles resulted if AgNO3 was used instead of Ag2SO4. if the Ag/Pt mole ratio was higher than 80% or lower than 20%, and if the degree of hydrolysis of the poly(vinyl alcohol) was higher than 98%.