Besides the high-tech sector, elements of the rare earth group are getting more and more attention in ferrous metallurgy. Cerium- and aluminium-containing inclusions can act as very effective nucleation sites for the heterogeneous solidification of steels. Therefore, the aim of this work was to characterize the formation of inclusions in the Fe-Ce-Al-O(-S) system. The literature part includes both a short introduction into the solidification behaviour of liquid metals, as well as an introduction into the mechanism and benefits of grain refinement of steels. Moreover, the first part is made up of a comparison of results of previous authors and some calculations with FactSage. Based on that, a total of thirteen experiments were carried out using a vertical tube furnace. Pure iron was used as base material. For alloying, Al-content was fixed at 100 ppm and Ce-content was varied between 39 and 280 ppm. Due to the high oxidation tendency of cerium, it was initially necessary to come up with the best possible alloying technique and furnace equipment in order to ensure a maximum output of the alloying elements. Using the final setup, samples were taken at five-minute intervals. Consequently, those were analysed regarding their chemical composition and their inclusion landscape. The comparison of the inclusions landscape with the calculated phase diagrams in equilibrium showed that a local supersaturation during alloying actually allowed the formation of unstable inclusions (such as Ce2O2S). In the course of the experiment these depleted of cerium and sulphur and took for other alloying elements such as aluminium. Various modification processes of the NMI caused by diffusion and phase transformations could be shown in the further course of the experiment. The system is moving towards larger sulphur free particles. The inclusions found could be assigned among others to the compounds Ce2O2S, AlCeO3 and Al11O18Ce.