The effect of MgO, Al₂O₃ and CaO/SiO₂ on the viscosity of CaO–SiO₂–Al₂O₃–MgO–5 wt-% TiO₂ slag was studied in the temperature range of 1673–1773 K. At a fixed CaO/SiO₂ ratio of 1·17 and 12 wt-% Al₂O₃, the viscosity of the slag decreased with increasing MgO content because of depolymerisation of the silicate structures. At a fixed CaO/SiO₂ ratio of 1·17 and 8 wt-% MgO, the viscosity of the slag increased with increasing Al₂O₃ content. At 8 wt-% MgO and 12 wt-% Al₂O₃ wt-%, increasing the CaO/SiO₂ ratio from 1·07 to 1·50 resulted in lower slag viscosity. The temperature dependencies of the viscosity on MgO addition, Al₂O₃ addition, and CaO/SiO₂ ratio were analyzed, and the apparent activation energies of each system were found to be between 178 and 232 kJ/mol, 273 and 360 kJ/mol, and 204 and 233 kJ/mol, respectively. Five different viscosity models were employed to predict slag viscosity, and the Riboud model was found to be the best for predicting this parameter.