Abstract: The effect of Al₂O₃ content on the viscosity and structure of CaO–SiO₂–Cr₂O₃–Al₂O₃ slags was investigated to facilitate recycling of Cr in steelmaking slags. The slags exhibit good Newtonian behavior at high temperature. The viscosity of acidic slag first increases from 0.825 to 1.141 Pa·s as the Al₂O₃ content increases from 0 to 10wt% and then decreases to 1.071 Pa·s as the Al₂O₃ content increases further to 15wt%. The viscosity of basic slag first increases from 0.084 to 0.158 Pa·s as the Al₂O₃ content increases from 0 to 15wt% and then decreases to 0.135 Pa·s as the Al₂O₃ content increases further to 20wt%. Furthermore, Cr₂O₃-containing slag requires less Al₂O₃ to reach the maximum viscosity than Cr₂O₃-free slag; the Al₂O₃ contents at which the behavior changes are 10wt% and 15wt% for acidic and basic slags, respectively. The activation energy of the slags is consistent with the viscosity results. Raman spectra demonstrate that AlO₄ tetrahedra appear initially and were replaced by AlO₆ octahedra with further addition of Al₂O₃. The dissolved organic phosphorus content of the slag first increases and then decreases with increasing Al₂O₃ content, which is consistent with the viscosity and Raman results.