The effect of MgO on the phase equilibria between matte and tridymite-saturated FeO_x-SiO₂-MgO slag was investigated using high-temperature equilibration and microanalysis technique. The results showed that at 1300 °C and p(SO₂) of 10 kPa, the presence of MgO reduced the FeO activity of the slag, leading to the decrease in the oxygen partial pressure (p(O₂)) over the slag and the Cu solubility in the slag. Due to the lower FeO activity, FeS was oxidized from matte into slag, increasing the Cu concentration of the matte. In high-SiO₂ concentration slags, excessive MgO in the liquid slag recrystallized as solid olivine (Mg₂SiO₄-Fe₂SiO₄), increasing the slag viscosity and copper losses in the slag. The maximum MgO solubility in the FeO_x-SiO₂-MgO slag increased from 3 wt.% to 8 wt.% when the temperature raised from 1200 to 1300 °C under p(O₂) of 10^-6 kPa. At higher MgO content, the slag composition could be optimized by increasing temperature and adding SiO₂ flux to maintain a fully molten slag or avoid high proportions of solids.