Interdiffusion between Pb and Pb-50wt% In (64 at.% In) alloy in the temperature range from 115 to 173°C has been studied using a bulk diffusion couple. Intimate bonding between the couple was achieved with the help of a soldering flux. Surfaces of the couple were finished by microtome and concentration profiles were determined by electron microprobe. The chemical interdiffusion coefficient. D ̃, was obtained by applying the Boltzmann-Matano solution to the profiles and the interface velocities, v. were measured by Kirkendall's technique using 2 mil dia. Al wires as interface markers. Following the analysis of Darken and Manning on D ̃ and v, we have determined the intrinsic diffusion coefficients of both Pb and In in a Pb-In alloy of 41 at.% In. We found that In diffuses faster than Pb by at least a factor of 2. The activation energy for In diffusion in the alloy is 15 kcal mole, which is considerably smaller than that of In in pure Pb, about 27 kcal/mole. Such a reduction in diffusion activation energy is anticipated in view of the much lower melting point of the alloy as compared to pure lead. Furthermore, the concentration dependence of D can also be adequately described by an empirical, melting point rule which utilizes the solidus temperature corresponding to a particular alloy composition. © 1976.