The effect of minority SrCO3, BaCO3, and SnO2 additions to various alkali carbonate electrolytes has been investigated for both oxygen reduction kinetics and also oxide ion activity. Although alkaline earth metal additives were found to assimilate into the melt with relative ease, metal oxides of higher valency were found to be fairly insoluble. In general, addition of earth metal carbonates to the electrolyte provides a means of increasing the effective oxygen solubility. The nature of the improvement appears to vary with the additive used, the amount added, and the original electrolyte composition. In all cases there was a significant increase in background current. In view of surface energy considerations, this is most probably due to increased carbon dioxide diffusion and meniscus effects. In addition to the increase in oxygen dissolution, there appears to be a concurrent increase in the oxide ion activity.