To obtain the fundamental information on the dissolution of nickel and copper into the slag formed in the nickel and nickel alloy smelting processes, the phase relations between Ni-Cu-Fe alloys and FeOX-MgO-SiO2slags saturated with MgO were investigated under controlled oxygen partial pressure using CO and CO2mixed gas at 1, 673 and 1, 773KThe MgO solubility in the FeOx-SiO2slag increased from 16 to 30 mass% corresponding to 10 and 40 mass%SiO2 respectively at 1, 773 K.Ni and Cu are concentrated in the metal phase while Fe is distributed mainly in the slag phase under the present experimental condition. The distribution ratios of Ni and Cu into the slag increase with increasing oxygen pressure. At 1, 673 K, Ni dissolution in the slag is lower than Cu atPO2less than 2×10-4Pa.In the higher oxygen potential region, the Cu dissolution in the slag is much higher than Ni. Similar behavior is observed between the distribution ratios of Ni and Cu at 1, 773KApplying the logarithmic relationship between the distribution ratio and oxygen pressure, nickel and copper form NiO and CuO0.5in the slag equilibrated with the ternary alloys.