Supermartensitic stainless steels possess an excellent combination of strength, toughness and corrosion resistance and have attracted an increased industrial attention especially from the offshore oil and gas industry, where those materials are already successfully in use. It is well known that the mechanical properties of this type of steels are strongly dependent on the fraction of retained austenite, which is controlled by heat treatment. Because the products manufactured out of these steels are in large sections, temperature gradients and corresponding compositional inhomogeneities are inevitable. Also during heat treatment partitioning of elements between the phases will give local concentrations far removed from the bulk levels. In the present work a 13Cr6Ni2Mo supermartensitic stainless steel is thermodynamically analyzed using the Thermo-Calc® software package where the influence of compositional variations on phase transformations is investigated, in particular the effect of changes in the Ae3-temperature is discussed.