Propagating surface plasmon polaritons excited on 49 at% Ag-Cu alloy thin films are studied experimentally. Co-sputtering technique is used to obtain metastable FCC solid solution phase and further annealed to achieve near equilibrium Ag-rich and Cu-rich phases. The dispersion of alloys is measured using the Kretschmann configuration. We show that the plasmonic response of alloy is sensitive to the phases formed during fabrication. Further, the annealed alloys show narrower dispersion compared to as-deposited alloys. © 2022 IEEE.