Highly stable gold–silver alloy nanoparticles with varying mole fractions were prepared in aqueous sodium dodecyl sulfate solution by simultaneous reduction of chloroauric acid (HAuCl₄) and silver nitrate (AgNO₃) using sodium citrate. The formation of alloy nanoparticles was confirmed by UV-visible spectroscopy and transmission electron microscopy. Particle size distribution was measured by dynamic light scattering. The surface plasmon absorption band of the Au–Ag alloy nanoparticles shows linear bathochromic shift with increasing Au content. Appearance of a single absorption peak in the visible region and lack of apparent core-shell structures in the transmission electron microscope images confirm the formation of homogeneous gold–silver alloy nanoparticles.