The first systematic measurements on the impact of interdiffusion between a metal overlayer and adhesion layer on the thermal interface conductance (G) at the metal bilayerdielectric interface are reported. Composition depth profiles quantify the interdiffusion of a Au-Cu bilayer as a function of Cu adhesion layer thickness (0-10 nm), annealing time, and annealing temperature. Optical pump/probe measurements of G quantify the effect of Au-Cu interdiffusion on thermal transport across the (Au-Cu)-Al 2 O 3 interface. The enhancement of G between Au and Al 2 O 3 through the addition of a Cu adhesion layer decreases as Au-Cu interdiffusion occurs. For example, annealing a 41 nm Au film with a 4.7 nm Cu adhesion layer on Al 2 O 3 at 520 K for 30 minutes, results in a 52 ±16% drop in G. An analytical model of the composition profile is derived with inputs of annealing time, temperature dependent permeabilities of the Au-Cu interface to each species, and the initial thicknesses of the Au and Cu layers. Integrating this model with a Diffuse Mismatch Model defines a new methodology for the prediction of G that accounts for interdiffusion in metal bilayers on dielectric substrates, and can be used to evaluate the degradation of G over a device's lifetime.