The viscosity of slags and casting fluxes is a key parameter in metallurgy. It influences the slag reactivity thus also the efficiency of metallurgical processes. In the continuous casting fluxes viscosity controls the lubrication, and subsequentially affects the quality of semi-finished products. The aim of this work is to investigate the impact of solid particles on the rheological behavior of melts. Therefor the influence of solid Zirconia (ZrO2) particles on the viscosity of a blast furnace slag as well as the viscosity of a casting flux with varying SiO2, CaO and Al2O3 content are analyzed at different temperatures. For the measurements, a high-temperature rheometer has been used. Different force modes (rotating and oscillating) as well as temperature programs (stepwise heating as well as cooling with a constant cooling rate) and shear rates were applied. The addition of ZrO2 to the blast furnace slag amounted 1, 3, 6, 10, 15 and 20 Vol.%. During the stepwise heating, the measuring took place isothermally at various temperature levels up to 1450°C. During cooling the viscosity was measured continuously. The experimental findings were compared and the dependence of the solid particle amount on the viscosity was characterized. Furthermore, a model to predict the rheological behavior in dependence of the solid particle amount, the viscosity of the partial melt and temperature was developed using experimental data. Starting with the original composition of the casting flux the SiO2 content has been additionally increased by 2 and 5 wt.%. Moreover, the Al2O3 (5 and 10 wt.%) as well as the CaO (2 and 5 wt.%) amount has been raised stepwise independently. Finally, 5 wt.% CaO plus additionally 5 wt.% Al2O3 have been added at the same time. All measurements took place under rotating and oscillating load conditions. The temperature has been increased stepwise up to 1400°C. The cooling was performed under a constant cooling rate of 5°C/min starting at 1400°C. The addition of oxides changed the viscosity properties according to their characteristic oxide network properties. In the end the experimental results were compared to viscosities estimated using thermochemical software FactSage, the equation proposed by Marsh and the viscosity module implemented in FactSage.