By the shear cell technique, the self-diffusion coefficient of tin in a liquid Sn-38.1wt% Pb alloy has been measured in the temperature range from 466°K to 786°K. The variation of DSn*Sn-38.1wt%Pbwith temperature doesn't follow the Arrhenius type equation and shows a change in its slope about 573°K. A qualitative explanation of this behaviour is attempted based on either a cluster hypothesis or rigid sphere liquid model.