Phase transformations and microstructural changes of a furnace cooled eutectoid Zn-Al based alloy were studied during ageing at 100 and 170degreesC using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Three phase transformations occurred in the furnace cooled eutectoid Zn-Al alloy. The metastable eta'(FC) phase decomposed during isothermal ageing. The four-phase transformation, alpha+epsilon-->T'+eta followed the discontinuous decomposition of the eta'(FC) phase. Typical morphologies of the decomposition of the eta'(FC) and epsilon phases were observed in scanning electron microscopy. Decomposition of Al-rich alpha phase was observed during the prolonged ageing by transmission electron microscopy. The different types of decomposition of the different metastable phases dominated at different stages of ageing. (C) 2003 Kluwer Academic Publishers.