Thin film specimens having the layered structure Pb/Ag/Au and Pb/Ag/Pd were prepared for a study of the reactions between Pb and Au, and Pb and Pd, respectively. These reactions require mass transport through the intermediate layer of Ag. In the case of the reaction between Pb and Au, it is found that after annealing at 200°C for more than 24 h the compound Pb2Au can be detected in those specimens where the 3000 Å thick Ag layer is polycrystalline but not in specimens where the Ag layer is a single crystal. This result indicates that grain boundaries in the polycrystalline Ag layer act as paths for rapid transport of Au and Pb atoms. In the case of the reaction between Pb and Pd, we prepared only polycrystalline Ag as the intermediate layer and detected the compound Pb2Pd after a few hours of annealing in the temperature range 160°-200°C. The lattice parameter of the polycrystalline Ag layer before and after annealing has been measured by X-ray diffraction and found to be unchanged, indicating that leakage into the Ag grains by lattice diffusion is negligible and that boundary transport is dominant. Separate study of grain boundary diffusion kinetics is now possible using this technique.