Tracer‐diffusion coefficients have been measured for Na+ and CO3−− ions in the pure melt and of Na+, K+, and CO3−− ions in a eutectic mixture of lithium, sodium, and potassium carbonates. The open‐ended capillary method was used for these measurements. In the melt, agreement with the Nernst‐Einstein equation is found at 910°C, but at 1050° there is a discrepancy of 22%. The experimental activation energies for diffusion of the ions in both the pure melt and the eutectic mixture are the same within ±1 kcal.