Both time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams have been established to characterize the crystallization behavior of slags in a selected temperature range. Therefore, the single hot thermocouple technique based on already existing equipment was constructed showing also some different approaches. Furthermore, the procedure was enhanced using a stretching device enabling the formation of very thin slag layers. This device permits the investigation of not only transparent but also translucent liquids with low amounts of coloring oxides, e. g. Fe(2)O(3). For the initiation of this method, a transparent synthetic NCAS-slag showing a high crystallization tendency was used to proper adjust the control parameters. Afterward, two industrial mould slags for the continuous casting of steel were investigated and TTT- as well as CCT-diagrams created. The TTT-diagrams for both mould slags show only one nose but the shapes of crystals formed differ dependent on temperature. This is contributed to the ratio of the growth to nucleation rate which is raised at higher temperatures where dendritic crystals are formed. For the case of continuous-cooling experiments observing only the formation of dendritic crystals precipitating at temperatures close to the liquidus temperatures the same explanation is assumed. Contrary, this ratio is decreased for rather low temperatures where only fine separate crystals precipitate.