Equilibrium ratios Cr2+/Cr3+of chromium oxide dissolved in CaO–chromium oxide–Al2O3–SiO2melts have been determined by analysis of samples equilibrated at 1500°C under strongly reducing conditions (po2= 10−9.56to 10−12.50atm). The majority of the chromium is divalent (Cr2+) under these conditions and Cr2+/Cr3+ratios at given constant oxygen pressures decrease with increasing basicity of the melts, expressed as CaO/SiO2ratios. In addition, Cr2+/Cr3+ratios, at a given CaO/SiO2ratio, are relatively unaffected by the amount of Al