Abstract: As a part of the fundamental study related to the reduction smelting of spent lithium-ion batteries and ocean polymetallic nodules based on MnO–SiO₂ slags, this work investigated the activity coefficient of NiO in SiO₂-saturated MnO–SiO₂ slag and Al₂O₃-saturated MnO–SiO₂–Al₂O₃ slag at 1623 K with controlled oxygen partial pressure levels of 10⁻⁷, 10⁻⁶, and 10⁻⁵ Pa. Results showed that the solubility of nickel oxide in the slags increased with increasing oxygen partial pressure. The nickel in the MnO–SiO₂ slag and MnO–SiO₂–Al₂O₃ slag existed as NiO under experimental conditions. The addition of Al₂O₃ in the MnO–SiO₂ slag decreased the dissolution of nickel in the slag and increased the activity coefficient of NiO. Furthermore, the activity coefficient of NiO (γ_NiO), which is solid NiO, in the SiO₂ saturated MnO–SiO₂ slag and Al₂O₃ saturated MnO–SiO₂–Al₂O₃ slag at 1623 K can be respectively calculated as γ_NiO = 8.58w(NiO) + 3.18 and γ_NiO = 11.06w(NiO) + 4.07, respectively, where w(NiO) is the NiO mass fraction in the slag.