Bimetallic AuAg (Au (80%) and Ag (20%)) nanoparticles (NPs) have been synthesized in fused quartz matrix using sequential ion implantation and post-annealing in air at 600 °C, 700 °C, 800 °C and 900 °C. X-ray diffraction (XRD) results confirm the formation of crystalline Au80Ag20 bimetallic NPs after annealing. Transmission electron microscopy (TEM) investigations have confirmed presence of spherical AuAg alloyed and few AuAg alloy core- Ag shell/satellite NPs. Rutherford backscattering spectra, along with fittings, revealed the mixing of Au and Ag atoms after annealing and movement of elements towards the surface. The peak position of the surface plasmon resonance (SPR) feature of Au has been shifted towards higher energy after sequential Ag implantation. The SPR peak position and intensity is further modified after annealing of Au80Ag20 NPs. The mechanism of modified SPR properties is mechanistically discussed by considering the diffusion, alloying, segregation and redistribution of Au/Ag in the host matrix.