AbstractA Zn–22 wt-% Al alloy was superplastically deformed in the optimal range to obtain experimental evidence based on scanning electron and transmission electron microscopy for mesoscopic grain/interphase boundary sliding and plane interface formation that facilitates the cooperative boundary sliding process. It was seen that such localised shear surfaces had formed in the entire specimen bulk in an interconnected manner. At boundaries that formed part of a cooperative shear surface the triple junctions had migrated to a near 1800 orientation. Mesoscopic grain/interphase boundary sliding appeared to be accommodated by local boundary migration brought about by diffusion at intercrystalline boundaries and lattice dislocation motion at interphase interfaces in accordance with the requirements of the model presented in Part 1.MST/3075