In the continuous casting process mould powders are added to the top of the liquid steel in the mould. There following layers are formed: a layer consisting of the original mould powder, a sintered layer and a liquid slag layer. The liquid slag is entrapped into the gap between the mould and the strand where a glassy, a crystalline and a liquid layer are present. To investigate the processes taking place in the mould different methods have been developed which partly show unsatisfactory or in some cases irreproducible results. Therefore in this paper the implementation of high temperature investigation methods and their valuation of suitability to characterise mould powders and slags are discussed. These methods are in one case the Hot Stage Microscope for the in-situ investigation of the sample during heating and cooling and in the other case the Hot Thermocouple Technique for the characterisation of the crystallisation behaviour of transparent slags. Using the Hot Stage Microscope during heating distinctive reactions like decarburisation or formation of a liquid phase of a pressed or pressed and polished mould powder sample can be observed. If a transparent liquid is formed also the temperature of first crystallisation including the shape of crystals can be determined. Based on the existing knowledge the effect of the addition of oxidic inclusions from the steel on the crystallisation behaviour of the original slag as well as the behaviour of the slag films taken from service in comparison to the one of the original sample was investigated. The Hot Thermocouple Technique was constructed based on the equipment of A. Cramb. A major difference between these methods is the separation of the measuring and heating circle. Furthermore two different programs using LabVIEW were developed: one for the temperature control of the sample and one for the data acquisition. For the initiation of this method a transparent synthetic NCAS-slag showing a high crystallisation tendency was used to proper adjust the parameters. Based on this knowledge industrial mould slags for the continuous casting of steel were investigated and Time-Temperature-Transformation- (TTT-) for 0,5% and 80% crystallinity and Continuous-Cooling-Transformation-diagrams (CCT-) created to characterise their solidification behaviour.