Grain-boundary diffusivities of a Sn119 radioactive tracer have been measured in polycrystalline Pb specimens in the temperature range 70-200°C. The combined diffusion parameter δDb is expressed by the Arrhenius relation δDb = 7.3×10-9 exp(-0.41±0.02 eV/kT) (cm3/sec). The Sn impurity diffusivities are larger than Pb self-diffusivities in the grain boundaries, which has been confirmed by codiffusion measurements of the two species. The lattice diffusivities reported in the literature for the Sn and Pb tracers also show a similar trend. In both situations the effect appears to be related to the physical characteristics of the two diffusing species.