NiO cathode dissolution is a serious problem with molten carbonate fuel cells (MCFC). The target life-time of such cells is 40,000 h, but shorting by NiO cathode dissolution markedly decreases cell performance. NiO cathode dissolution depends on the composition of the molten carbonate electrolyte. The electrolyte generally comprises a mixture of lithium carbonate and potassium carbonate. Since the solubility of NiO in a mixture of lithium carbonate and sodium carbonate is lower than in lithium and potassium carbonate, it is expected that shorting by NiO cathode dissolution will take longer in a mixture of lithium and sodium. Therefore, a mixture of lithium carbonate and sodium carbonate is a strong candidate electrolyte. A unique 10-kW class stack, which uses a mixture of lithium and potassium carbonate and mixture of lithium and sodium carbonate as the electrolyte, has been developed and tested. The basic performance and life time of both electrolyte cells of the stack are reported. In particular, the change in cathode polarization caused by NiO cathode dissolution is evaluated quantitatively.