The spacing of grain boundaries at which grain boundary sliding (GBS) had occurred during superplastic (SP) deformation was determined by measuring the length of segments of marker lines inscribed on the pre-polished surface in Pb-62%Sn after superplastic deformation in shear. Statistical distribution of this segment length (L) was bimodal at low strain levels, but became unimodal at high strain levels. The concept of cooperative GBS, i.e. sliding of groups of grains as an entity, has been invoked to explain the evaluation of the L-distribution with strain. This investigation suggests that the real spacing of sliding grain boundaries should be taken into account for modelling of SP flow.