Some results on the lateral diffusion of indium in thin lead films containing 2.5 wt.% Au are described. At room temperature the diffusion rate is high with a diffusion coefficient D of about 2.5 x 10-12 cm2 s-1 and an activation energy of 0.26±0.1 eV. The indium concentration profile along the films is highly irregular and is characterized by a sharp peak within the diffused film area at the diffusion front. Some of the irregularities are explained by the formation of filaments of near-stoichiometric AuIn2 and by diffusion around grains of varying size. The preferential formation of AuIn2 is likely to be the cause of the lower diffusion rate observed in the Pb-Au films in comparison with pure lead films. It is found that hillocks are nucleated in the vicinity of the indium diffusion front, indicative of strain relaxation. © 1977.