In the continuous casting process of steel interactions between refractory, molten steel and mold slag take place. Due to the absorption of contaminating oxides from the refractory and the molten steel the chemical composition of the mold flux changes. Therefore the characterization of a mold slag – with the addition of TiO2, ZrO2, MnO and Al2O3 each with 1%, 2%, 5% and 10% – and its slag films was the subject of this thesis. The Hot Stage Microscope (HSM), Simultaneous Thermo Analysis (STA), Crucible Crystallization Experiments (CCE), Reflected-Light Microscope, Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) were used to determine the crystallization behavior. The original mold slag and the mold slag with the addition of 1% as well as 2% TiO2 were also analyzed using the Single Hot Thermocouple Technique (SHTT). The in-situ observations with the HSM and the SHTT allowed the identification of the crystallization temperature and the crystal shapes, resulting mostly in needles and subsequently in dendrites. Distinctions could mainly be observed for higher amounts of oxides. Samples with 10% ZrO2 and 10% MnO were not analyzed using the HSM. The ZrO2 could not be dissolved due to its solubility limit in the slag; higher amounts of MnO resulted in an opaque slag. The addition of TiO2 led to the crystallization of cone like and the addition of Al2O3 to cubic and columnar shaped crystals. Cubic and cone like crystals could also be observed within the slag films showing dendritic growth. Mineralogical investigations confirmed these results and allowed the analysis of the phase composition. The main phases turned out to be Cuspidine and a glassy solidified slag. Additionally Pyrolusit (ß-MnO2) in the sample with 5% MnO addition, Perovskit (CaTiO3) in the one with 10% TiO2 addition and Orthoclase (K[AlSi3O8]) in the samples with 2% up to 10% Al2O3 addition were detected. The experimental results obtained by STA and CCE showed good correlation, even though the evaporation of alkali oxides and fluorides due to the chosen temperature program with the STA led to higher crystallization temperatures. Samples with the addition of MnO as well as Al2O3 and TiO2 showed a decreasing crystallization temperature with increasing addition, while the ones with the addition of ZrO2 resulted in an increase of the crystallization temperature. The determination of the crystallinity in dependence on time was carried out with SHTT (TTT- and CCT- experiments) and resulted in elongated crystallization duration with increasing TiO2 addition. The slag films were analyzed using HSM, STA and CCE, showing similar crystallization behavior and merely minor differences in the crystallization temperatures. However, they differed significantly from the original mold slag concerning first crystallization by forming cubic and cone like crystals, whereas the original mold slag crystallized in the shapes of needles and columns. The slag films also showed lower crystallization temperatures than the original slag. In addition to the chemical analysis these results indicate the changes of the slag based on the interactions with the steel bath and the refractories, which emphasizes the importance of these studies.