The residual‐activity method was employed to evaluate the volume and grain‐boundary diffusion coefficients of mercury in the Ag3Sn alloy at temperatures ranging from 50 to 245 °C. The volume diffusion coefficient obeys the relationshipDv*=1.87×10−5 exp(−8.25×103/RT) cm2/sec. The grain‐boundary diffusion coefficient evaluated from Suzuoka’s exact solution varied from 10−3cm2/sec at 50 °C to 10−1cm2/sec at 245 °C. The large grain‐boundary‐diffusion coefficient was attributed to the increased grain‐boundary width of the alloy caused by the corrosive nature of the mercury.