We report on the formation of Au-Ag nanoalloy by laser ablation of Ag plate in the Au nanocolloidal solution. Development of an intermediate phase of Au core/Ag shell nanostructures is noticed at the initial stage of alloying which is evident from the optical extinction spectra and is further confirmed from Transmission Electron Microscopic image. The intermediate phase is found to be stable and continued ablation of Ag target resulted in the formation of nanoalloys with plasmon band tunable depending on the time of ablation. (C) 2015 Elsevier B.V. All rights reserved.