The effects of La, Rb and Ba additions to (Li+Na)₂CO₃ 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)₂CO₃ melt at 650^oC were examined using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CD^0.5 values determined from CV and EIS were not comparable. The addition of 0.5 mol% of La₂O₃ to (Li + Na)₂CO₃ showed a remarkable increase in CD^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)₂CO₃ as electrolyte and operated at 650^oC were examined using I-V curve. The total voltage loss was decreased by 13% when 0.5 mol% of La₂O₃ was added to (Li + Na)₂CO₃ electrolyte but was decreased by 9% when 3 mol% of Rb₂CO₃ was added to (Li + Na)₂CO₃.