Direct observation in a scanning electron microscope of the evolution of the grain-boundary sliding (GBS) process in a Pb-62wt%Sn eutectic alloy during superplastic deformation in shear is reported. The distribution of GBS along the shear surface is found to be inhomogeneous and there is evidence of coupling of the processes of GBS and grain-boundary migration. The rate of GBS increases at small stages of strain (up to about 0·8) and decreases thereafter, indicating that the GBS process is eventually accompanied by strain hardening. The observations are compatible with the dislocation model for GBS.