Phase transformations and microstructural changes of an as-cast eutectoid Zn-A! alloy (ZnAl22Cu2) were investigated during isothermal holding. The typical dendritic structures consisted of alpha’(s) phase as a core with the edge of decomposed beta’(s) phase and decomposed eta’(s) in the interdendritic regions. A series of complex phase transformations was observed. Both decompositions of beta’(s) and eta’(s) were determined at an early stage of ageing and a four-phase transformation, alpha(f)+epsilon --> T’ + eta, was observed at the boundaries of alpha(f) phase and the epsilon phase, instead of clearly observed at the boundaries of epsilon phase, in a solution-treated Zn-Al alloy during prolonged ageing.