Using sol-gel processing and high-temperature annealing, metal alloy nanoclusters of AuCu and Au4Cu in SiO2 thin films were synthesized. The crystallographic data show that the lattice parameters of those nanocluster alloys, both showing the fcc structure, are very close to the bulk values. The nanocrystals obtained by annealing at 900 degrees C were nearly spherical in shape, having the AuCu size of ca. 23 nm in radius and the Au4Cu size of ca. 35 nm. The peaks of the optical spectra of the samples annealed at 700-900 degrees C are located at 555 nm for AuCu and at 562 nm for Au4Cu.