In this study, the effects of La, Rb and Ba additions to (Li+Na)CO₃ electrolyte were investigated using a half-cell and a coin-type MCFC. The effect of La and Rb on oxygen reduction on a gold electrode in a (Li+Na)CO₃ melt at 650^oC were examined using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The C₀ D₀^0.5 values determined from CV and EIS were not comparable. The addition of 0.5 mol% of La₂O₃ to (Li + Na)CO₃ showed a remarkable increase in C₀ D₀^0.5 values determined by both EIS and CV analyses. The effect of La and Rb on cell performance of coin cells using (Li+Na)CO₃ as electrolyte and operated at 650^oC were examined using I-V curve. The total overpotential was decreased by 13% when 0.5 mol% of La₂O₃ was added to (Li + Na)CO₃ electrolyte but was decreased by 9% when 3 mol% of Rb₂CO₃ was added to (Li + Na)CO₃.