The mechanism of the interaction of polycrystalline Ag-Al thin-film couples to form Ag2Al at temperatures from 107 to 221°C has been studied. Techniques of film characterization included Rutherford backscattering, x-ray diffraction, and SEM. The dependence of the interaction on the grain size of the Ag film (660-7500 Å) was measured. An effective activation energy of 0.86±0.05 eV was found. The reaction is believed to occur initially by nucleation and growth of Ag2Al grains at Ag grain boundaries at the interface; after formation of a continuous Ag2Al layer, the growth of this layer follows a t1/2 law, controlled by diffusion through the grain boundaries of the Ag2Al layer.