Nickel oxide cathode solubility in the molten carbonate fuel cell (MCFC) electrolyte has been identified as one of the major technical obstacles facing fuel cell commercialization. The solubility of transition metal oxides is thought to be a function of the acid/base chemistry of the melt, controlled by the partial pressure of the oxidant gas. In this study, equilibrium nickel oxide solubility experiments were performed in a molten mix under various gas compositions to identify the conditions of minimum solubility. An analytical procedure is presented in which the dissolved nickel ions are isolated from the frozen electrolyte and spectrometrically analyzed to a detection limit of 0.5 ppm (μg Ni/g electrolyte). Under current MCFC operating conditions, the partial pressure is such that cathode solubility is found to occur under acidic dissolution conditions. For a given gas atmosphere, it is shown that solubility can be reduced by the addition of small amounts of basic alkaline earth oxides to the acidic electrolyte. The addition of 1 weight percent resulted in a fifteenfold decrease in the dissolved concentration.