The aim of this work was the investigation of the viscosity and the break temperature of 11 mould powders for the continuous casting of steel in the temperature range of 900-1400°C by means of a rotational viscosimeter of the company Bähr Thermoanalyse GmbH. First the device-specific correction factors for the viscosity and the sample temperature were determined and the procedure for the measurement of the mould powders was optimised. The determination of the correction factors was performed with standard reference glasses as well as with a standard reference oil and by using an external thermocouple. Then the mould powders were characterised by their viscosity and break temperature. The experimental results were compared with calculated data. Calculation models which had been developed for slags of mould powders were taken from literature, e.g., the formula by Riboud and the one by IRSID for the viscosity and the formulae by Kim and Sridhar et al., respectively, for the break temperature. The results show, that a comparison of the experimental and the calculated data is only possible to a certain extent, since the models were developed for a specific range of validity for the chemical composition of mould powders. In addition to that a possible change in the composition of the mould powders during the proceedings of the measurement were not regarded; only the composition of the mould powders as received was taken into account.