Systematic investigations on microstructures and phase transformations were carried out on solution treated, chilled as-cast and extruded Zn-Al based alloys (containing small amounts of Cu or Cu and Si). Phase transformation sequences of the thermal and thermomechanically treated eutectoid Zn-Al alloys were studied along with isothermal holding. Two new unstable phases eta (S)' and eta (E)' and their decomposition products in the chilled as-cast and extruded eutectoid Zn-Al alloys are discussed.