Viscosity and melting behavior 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 during tapping or infiltration of refractory while too viscous liquids can lead to problems regarding the smelting of charged material or metal inclusions. This paper comprises the determination of melting behavior and viscosity of slags as functions of their composition. Therefore, 19 mixtures of different oxides have been prepared and melted by means of an induction furnace. An analysis of the resulting slags in a hot stage microscope and the computation of the corresponding viscosities allow an evaluation of the most important parameters. Significant variations of viscosity and characteristic temperatures can be measured as functions of the content of manganese oxide and the ratio between calcium oxide and silicon dioxide, respectively, besides a strong temperature dependency and a major influence of network-forming oxides.