[en] Reliable thermodynamic data are essential for the design and the production of new alloying systems. Particularly, the knowledge of the high-temperature phase transformations (T_Liquid, T_Solid, T_Perit, T_γ→δ) are important for the solidification and the further processing. Investigations of selected commercial Dual-Phase, TRIP and high-Mn TWIP steels by DTA/DSC measurements show that the experimental results differ significantly from the calculation results of thermodynamic databases with respect to the phase transformation temperature and sequence. Based on these findings, it is very important to identify the defective subsystems of complex alloys in order to optimise the thermodynamic databases. In order to verify a quaternary system, e.g. the Fe-C-Si-Mn system, it is important to check the corresponding ternary subsystems. This was performed by DSC measurements of selected model alloys. By doing so, it was found that in Si- and Mn-alloyed Dual-Phase steels the thermodynamic description of the Fe-Si-Mn system is currently inadequate. This is a very important result, since all new designed steel grades for the automotive industry are based on a Fe-C-Si-Mn matrix. (paper)