Basicity of the (Li0.62K0.38)2CO3, the current choice of electrolyte composition for molten carbonate fuel cells (MCFC's ), is defined as — log (a M 2O), where M represents an alkali metal and a M 2O is the net oxide ion activity. Net oxide ion activity is defined as the sum of the alkali oxides activities dissolved in the melt. To correlate measured cell e.m.f. values with basicity change in the (Li0.62K0.38)2CO3 melt, a dual electrode galvanic cell of the following arrangement was tested at 650°C with P CO 2varying above the melt: Au, A—B, CO2, O2 | mullite | A—B, CO2, O2 | ZrO2 · Y2O3 | O2, Au where A—B represents (Li0.62K0.38)2CO3. The response of the cell to P CO 2 at constant P O 2 can be explained by thermodynamic model, which states that ion transference in the mullite tube is limited to Li* and/or K* and the dual electrode galvanic cell voltage is a direct measure of Δa Li 2O or Δa K 2O for pure (Li0.62K0.38)2CO3 melt at constant P O 2.