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Austempered ductile iron (ADI) proved to be an advanced material as it possesses remarkable combination of high strength, ductility and toughness with good wear resistance and machinability. These properties can be achieved upon adequate heat treatment which yields optimum microstructure for a given chemical composition. In the present paper, an investigation has been conducted on unalloyed ADI and alloyed with 0·47%Cu and 1·6%Cu + 1·5%Ni, austempered at 350°C for the time up to the 6 h. The microstructure and fracture mode developed throughout these treatments have been studied by means of light, transmission and scanning electron microscopy, as well as X-ray diffraction. It was shown that the strength, elongation and impact energy strongly depend on amount of ausferritic ferrite and stable, high carbon enriched retained, reacted austenite. Based on the results, optimal processing windows for alloyed ductile irons used, have been established. In addition, for alloyed ADIs with maximum amount of reacted, retained austenite, transition temperature was also obtained.