A novel hydrothermal layer-by-layer processing method for the fabrication of core/alloy nanoparticles with highly tunable surface plasmon resonance is described. For a model system of Au/AuxAg1-x, the processing temperature, alloy composition, and alloy thickness resulted in unique and tailorable plasmonic signatures. The discrete dipole approximation and selective alloy etching were used to correlate this optical response with the particle morphology and alloy phase ultrastructure.