AbstractMicrostructural observations and measurements of the retained austenite content, hardness, austenite carbon content and unreacted austenite content are reported as a function of austempering time at 400, 375, 350, and 300°C after austenitising at 920°C for a ductile iron of composition (wt-%) Fe–3·49C–2·33Si–0·42Mn–0·004P–0·016S–0·25Cu–0·23Mo–0·035Mg. Solute profiles are presented which show solute segregation during solidification and that austenitising for 8 h does not remove the segregation. The segregation of Mo and Mn to intercellular areas is shown to result in stage I austempering reactions in the eutectic cell and intercellular region occurring at different austempering times. Evidence is provided of the occurrence of the stage II austempering reaction before the completion of the stage I reaction. The consequence of this sequence of changes is that the processing window is predicted to be closed for austempering temperatures of 400 and 375°C and open for temperatures of 350 and 300°C.MST/3392