Describing the conditions for reaustenitization of spheroidal graphite cast irons is of interest for their heat-treatment after casting, e.g. for manufacturing austempered ductile irons. Differential thermal analysis has been used to characterize the direct eutectoid transformation and the reverse transformation, i.e. the reaustenitization. This has been applied to a standard and a Ni-bearing alloy, with a ferritic matrix for the former, both a ferritic and a pearlitic matrix for the latter. The results are discussed in relation with the stable and metastable three phase fields. While earlier description of the direct eutectoid transformation is confirmed, the one for reverse eutectoid has been found more complex and is amended.