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Ageing characteristics of an extruded eutectoid Zn-Al based alloy were investigated using X-ray diffraction and scanning electron microscopy techniques. The extruded alloy consisted of Al rich α phase and Zn rich η′E and ε phases. The original cast eutectoid Zn-Al alloy was extruded at 250 °C. Both supersaturated α′s and β′s phase decomposed during extrusion and appeared as fine and coarse lamellar structures. The η′E and ε phases particles formed in the original interdendritic region. It was found that two Zn rich phases η′E and ε decomposed sequentially during ageing at 170, 140 °C. The decomposition of the η′E phase occurred as a discontinuous precipitation in the early stage of ageing and the decomposition of the ε phase took place in a four phase transformation: α + ε → T′ + η in the prolonged ageing. Two typical morphologies of the decomposition of the Zn rich phases η′E and ε were distinctive in back-scattered scanning electron microscopy.