The effect of three different solutionizing and annealing heat treatments on the initial superplastic microstructure of a Zn-21Al-2Cu alloy was studied. Microstructural characterization using SEM and DRX techniques revels the presence of residual grain boundaries of a high temperature β phase (referred to as Former β Boundaries, FβBs) which encompass groups of fine α (Al rich phase) and η (Zn rich phase). The origin of these arrangements may most likely be due to small deviations from the eutectoid composition as consequence of the Cu addition, which preferentially promotes the precipitation of proeutectoid phase to the high temperature phase boundaries. The final domains size only are influenced by the solutionizing treatment time, while the fine phases size only is affected by the duration of the subsequent annealing process. The presence and characteristic of his FβBs could be import in order to explain possible microstructural changes, which lead to a reduction of the grain boundary sliding capacities of fine α, and η phases that conduces at the early onset of a non-stable plastic flow previously observed in this alloy.