Due to the excellent combination of mechanical properties, high manganese steels are one of the major trends in materials development. These full or partial austenitic steels contain up to 30 wt% manganese and in many alloys the aluminium concentration is up to 12%. Beside the materials development, it is also of upmost importance to investigate the metallurgical fundamentals for the production of these grades. The work at hand is focused on the most important aspects related to the secondary metallurgy of high manganese steels. One of the key aspects of the experimental investigations is the solubility of oxygen in high manganese steels with or without increased aluminium contents. In this context, the equilibrium Fe-Mn-O is restudied for concentrations up to 20% Mn. Following the Wagner formalism, the first order interaction parameter, the equilibrium constant and are determined and the deoxidation products are analysed. The investigations of the oxygen solubility in the systems Fe-Al-O and Fe-Mn-Al-O are carried out in a newly developed experimental set-up for the determination of equilibria at elevated temperatures. One of the main results is that there is no sudden increase in the oxygen solubility at higher concentrations of alloying elements. This finding is obviously contradictory to the state-of-the-art in research. With increasing manganese content, the nitrogen solubility in the liquid state is rising. For this reason, nitrogen plays a crucial role in the secondary metallurgy of high manganese steels. Apart from the well-known method of vacuum treatment to decrease nitrogen in liquid steel, the possibility of lowering the nitrogen content by means of slag treatment is investigated. In the case of extremely low oxygen activities in the metal phase, which can be provided by high aluminium contents, it is possible to decrease nitrogen by metal-slag-reaction. This happens even before the nitride precipitation is decisive for the nitrogen solubility. Aside from the metallographic work on the oxides originated from the equilibrium experiments, nitrides are of special concern within this work. The morphology of the nitrides AlN, TiN and ZrN is characterized. It is evident that the morphology is mainly affected by the point of precipitation and the supersaturation conditions.