It is common for researchers to believe that liquid slag is a fluid characterized by rheological behavior of ideally viscous Newtonian body. At the same time it is assumed that the fluid is similar to synthetic polymers in terms of its structure. The authors of this paper conducted research into 4-component liquid and solid-liquid synthetic slag system: CaO–SiO2–MgO–Al2O3 The research involved graphite measurement systems. A perforated spindle was used for the solid-liquid systems subject to shear stress, whereas a smooth one was used for fully liquid systems. Both measurement systems worked according to Searle’s system. This paper presents selected rheological research results concerning oxide solutions in high temperatures and their rheological analysis.