Controlling austenite formation kinetics and its mean chemistry during annealing are two ways to act on the in-use and mechanical properties of high-carbon high-formability steels. Indeed, these characteristics influence austenite stability at room temperature after cooling and are therefore at the origin of potential TRIP effects under mechanical loading. Atom probe tomography analyses and Thermo-Calc/DICTRA calculations were used to understand how austenite forms during 1 degrees C/s heating of a 0.2 wt% C high-formability steel. The proposed analysis is conducted in the quaternary FeCMnCr system. It is shown that the transformation kinetics can be rationalized from the evolution of interfacial compositions, which are imposed by the operative tie-lines positions in the relevant phase diagrams. This analysis method helps to gain insight on the evolution of phases chemistries upon heating. Good agreement is found between experimental and calculated kinetics, but also between APT-measured and calculated phases chemistries. Mn and Cr partitioning between ferrite and austenite is confirmed during heating. APT also allows for the characterization of segregation on alpha/gamma interphase boundary, showing that Mn segregation already occurred at 750 degrees C.