Ferrous calcium silicate (FCS) slags are reported to have high potential for use in the copper converting process. However, no phase equilibrium study has been carried out under a simulated converting gas atmosphere in the presence of SO2. In this study, under accurately controlled P(SO2) 0.4 atm and P(O-2) 10(-6)atm, the phase equilibria of FCS slags in a liquid/spinel/white metal/gas system were investigated for the copper converting process. The liquidus temperatures of slags in the Fe-Ca-Cu-Si-S-O system were quantified as well as the solid solubilities of Cu2O in the spinel phase. The experimental results were compared with the FactSage predictions, which provided clues for further improvement of the thermodynamic modelling. The applications of the FCS slags were further discussed and compared with iron silicate slags. The results obtained will contribute to slag development for converting operations and thermodynamic calculations.