Abstract: We investigated the effect of Al₂O₃ content on the viscosity of CaO–SiO₂–Al₂O₃–8wt%MgO–1wt%Cr₂O₃ (mass ratio of CaO/SiO₂ is 1.0, and Al₂O₃ content is 17wt%–29wt%) slags. The results show that the viscosity of the slag increases gradually with increases in the Al₂O₃ content in the range of 17wt% to 29wt% due to the role of Al₂O₃ as a network former in the polymerization of the aluminosilicate structure of the slag. With increases in the Al₂O₃ content from 17wt% to 29wt%, the apparent activation energy of the slags also increases from 180.85 to 210.23 kJ/mol, which is consistent with the variation in the critical temperature. The Fourier-transform infrared spectra indicate that the degree of polymerization of this slag is increased by the addition of Al₂O₃. The application of Iida’s model for predicting the slag viscosity in the presence of Cr₂O₃ indicates that the calculated viscosity values fit well with the measured values when both the temperature and Al₂O₃ content are at relatively low levels, i.e., the temperature range of 1673 to 1803 K and the Al₂O₃ content range of 17wt%–29wt% in CaO–SiO₂–Al₂O₃–8wt%MgO–1wt%Cr₂O₃ slag.