The Molten Carbonate Fuel Cell (MCFC) is a promising option for highly efficient generation of electricity. The overall objective of this study was to optimize the composition of the presently used alkali carbonate electrolyte by modifying important fuel cell performance and endurance-related properties. A short list of carbonate melt characteristics affected by changes in alkali composition includes: ionic conductivity, vapor pressure and melting range, gas and electrode solubilities, materials corrosion and matrix stability effects, as well as surface tension, diffusion and electrokinetic effects. The study focused primarily on out-of-cell corrosion, solubility and stability of components, and electrode kinetic and wetting effects of the various candidate electrolyte compositions selected for screening. Finally, bench-scale (300 cm{sup 2}) fuel cell tests were operated, lasting from 1000 hours up to 5000 hours to confirm the general conclusions reached from the literature data and out-of-cell experiments. Overall, the major accomplishment of this study has been to show that significant improvements in fuel cell operation may be obtained by employing the eutectic lithium/sodium carbonate composition as the MCFC electrolyte. 84 refs., 26 figs., 13 tabs.