Slags are important in iron and crude steel making as well as in secondary metallurgical treatments. Slag properties, such as viscosity, density, liquidus temperature, oxidation potential, foaming behavior, etc., have a significant influence on the efficiency of metallurgical processes. These properties are largely defined by the slag composition. The slag composition is adjusted in all metallurgical processes by the addition of additives. These are usually CaO- and/or MgO-containing additives. Knowledge of diffusion coefficients of the respective oxide in slags is necessary to describe dissolution processes of additives in slags and therefore to optimize existing processes or develop new ones. Furthermore, the knowledge of thermodynamic activities of individual slag components is of great interest for understanding and optimizing metallurgical processes. A low activity of the species to be extracted (e.g. P or S) in the slag leads to an efficient removal of these components from the steel. The capacity of slags to absorb unwanted steel by-elements is influenced by the composition of the slag. The determination of these two parameters (diffusion coefficient and activity) in slags was the objective of this work. For the investigation of diffusion coefficients in slags, an experimental setup was designed which allows dissolution experiments to be carried out under reproducible conditions. In addition to the construction of the experimental setup, a calculation model was developed, which enables the calculation of the diffusion coefficient from the data of the dissolution tests. This model includes a density model and five different viscosity models. Since each viscosity model is valid for a certain temperature and composition range, it was necessary to use several models to investigate different slags. Besides the design of the experimental setup and the development of a suitable calculation model, a device as well as procedures for CaO sample production had to be planned. The diffusion coefficients of CaO and MgO in a blast furnace, calcium aluminate and wollastonite slag at 1500 and 1600 °C, and in a BOF slag at 1400 and 1500 °C were obtained by various dissolution experiments. Additionally, the diffusion coefficients or dissolution rates of Al2O3, SiO2 and MgAl2O4 were measured in a ladle furnace slag at 1550 °C. The results and the comparison with literature data showed in general a decrease of the diffusion coefficient with increasing slag viscosity. Comparison with literature data also confirmed the suitability of the experimental setup and the calculation model. A method allowing the determination of activities of individual slag components without the need for literature data was required. Such a procedure could be realized using Knudsen effusion mass spectrometry (KEMS). The KEMS measurements were carried out at Forschungszentrum Jülich. Since this method was rarely used in the metallurgical research field, the experiments, in the course of this work, were able to establish a basis for further research. In addition to the determination of the activity of CaO and MgO in various slags, the KEMS measurements also allowed the calculation of the enthalpies of formation of both oxides. Comparison of the measurements with literature values confirmed the suitability of this method for activity measurements of slag components.