Cu/Al bilayer reactions have been studied by Rutherford backscattering, Auger depth profiling, transmission electron microscopy (TEM), and internal friction. In the later stage of the solid‐state reaction Al2Cu Θ phase growth occurs in a planar manner at the top of the Al film and additionally along the grain boundaries of the columnar Al grains. For the early stage of the reaction where only small amounts of Cu have diffused into the Al, internal friction proved to be the only experimental technique sensitive to Cu concentrations below 500 ppm. It was found that 300 ppm of Cu are already sufficient to shift the grain boundary relaxation peak observed by internal friction to higher temperatures compared to pure Al. This behavior reflects a rise in activation energy for grain boundary diffusion from 0.55 eV for Al to about 1 eV for the AlCu system. At higher Cu concentrations the grain boundary relaxation peak remains unaffected up to a Cu concentration of 5%, decreases for higher concentrations and is completely suppressed at 19%. This behavior is closely related to microstructural changes at the grain boundaries as observed by TEM.