Using recent and more accurate data, the thermodynamic model of Ma and Janke has been extended to a five element system. The content of non-metallic inclusions has been investigated for two grades of duplex stainless steels after solidification. The chemical and morphological features of the inclusions were determined by SEM observations and EDS analysis for different cooling rates. The results are interpreted on the basis of the chemical-physical interactions in the Fe-Cr-Ni-Si-Mn-O system. They can be used to define the optimal oxygen potential pattern for the casting of this type of steel in foundries.