The microstructural characteristics of ductile cast iron austempered for various times are characterized and correlated to Charpy impact toughness. The desired matrix structure of austempered ductile cast iron consists of a two-phase mixture of carbide-free bainitic ferrite and austenite. During austempering, bainitic transformation can be described as a too-stage reaction : 1) austenite decomposition to bainitic ferrite and carbon-enriched austenite, followed by 2) decomposition of carbon-enriched sustenite to further ferrite and carbide. Therefore, excessive austempering produces a sharp drop in impact toughness and a fracture mode transition from ductile tearing to brittle quasi-cleavage fracture by a carbide formation. Delaying the embrittling reaction by addition of Ni and Mo should allow a greater latitude in austempering time. However, the onset of the embrittling reaction cannot be detected metallographically. By the way, interruption of the austempering reaction by quenching prematurely will result in a formation of martensite. This also reduces impact toughness.