The solubilities of lithiated NiO produced by in situ oxidation of Ni porous plaques have been measured in (62 + 38)mol.%(Li + K)CO3 melt under pressurized conditions as functions of partial pressures of CO2 and O-2, and of temperatures. The high performance liquid chromatography technique was applied to the solubility measurements. It was found that the NiO solubility was proportional to the CO2 partial pressure (0.1 less than or equal to P-CO2 less than or equal to 3.6 atm, P-O2 = 0.9 atm) and independent of the O-2, partial pressure (0.1 less than or equal to P-O2 less than or equal to 3.6 atm, P-CO2, = 0.9 atm), and decreased with increasing temperature from 823 to 973 K under a total pressure of 5 atm of air +CO2, (0.7:0.3 oxidant gas). The results are explained in terms of the acidic dissolution mechanism at the acidic side (P-CO2, greater than or equal to 0.1 atm). However, it is noted that the solubility obtained under 5 atm of air +CO2 (0.9:0.1) was independent of temperature.