BothImpurities management primary and secondary copper production industries have been facing strong needs to treat tinTin-bearing materials to enhance the sustainability of societies worldwide. There has, thus, been a growing concern over the tinTin behavior in the copper converting stage in which the operators must avoid the precipitation of both magnetite and refractoryRefractory stannic oxide. It is reasonable to use the lime-ferrite slagLime-ferrite slag of the CaO-FeOx-Cu2O system to avoid the precipitation of magnetite under an oxidizing atmosphere. The present paper determined the solubility of SnOx in the lime-ferrite slagLime-ferrite slag. These have been experimentally deduced from liquidus isotherms of the CaO-FeOX-SnOY-Cu2O system coexisting with molten copper under various oxygen pressures between 10–9 and 10–5 atm at 1300 ℃. The relations between the Sn solubility and the slag composition, and the distribution of Sn between slag and molten copper against the oxygen pressure of the system will also be presented to discuss feasible copper smelting/converting operations.