The influence of 0.02% and 0.1%Mg on the superplastic properties of the quenched Zn-22 wt%Al eutectoid alloy was investigated between room temperature and 250°C. Even small amounts of Mg raise the flow stress markedly and reduce the elongation to fracture. In the 0.1Mg alloy the decomposition process after quenching does not produce the usual fine structure but large Al grains containing finely dispersed Zn particles. This material is brittle up to 100°C, but shows superplastic properties at higher temperatures. It is observed that at such elevated temperatures the initial coarse grains are replaced by a finer structure during deformation. In all alloys it is demonstrated that on superplastic straining an initial short region of strong work hardening exists. From structure changes during deformation it is concluded that an extensive grain boundary movement occurs and that this is an essential factor for superplastic deformation. These results can best be explained by the assumption that the superplastic phenomenon is due to the continuous production of strain free grains by grain boundary migration.