Viscosity and melting behaviour are two of the most important properties of industrial slags as they directly impact the yield of metallurgical processes. Too high fluidity may cause losses while tapping too viscous liquids can trigger problems regarding the smelting of charged material. This thesis comprises the determination of a correlation between melting behaviour and viscosity of slags and their composition. Special focus lies on the impact of the content of manganese oxide. In order to find answers on these scientific problems various mixtures of different oxides have been prepared and fused by means of an induction furnace. A subsequent analysis of the resulting slags in a hot stage microscope and the computation of the samples’ viscosities allow a comparison of results and an evaluation of the most important parameters. The conclusions of the executed tests show significant differences between viscosity and characteristic temperatures as functions of the MnO content besides a strong temperature dependency and a major influence of network-modifying and network-forming oxides. The viscosity is decreasing steadily with increasing percentage of manganese oxide while flowing and hemisphere temperatures show upward tendencies.