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The weak association between sodium and carbonate ions has been investigated at 25°C using high-precision sodium ion-selective electrode potentiometry in solutions of ionic strength ranging from 0.5 to 7.0 M in CsCl and in 1.0 M Me4NCl media. The protonation constants of CO2−3 (aq) were also measured, using a H+-responsive glass electrode in 1.0 M Me4NCl and NaCl. The value of the ion-pair association constant calculated from the difference in the protonation constants in these two media was in excellent agreement with that obtained from the Na+ISE measurements. Evidence is also presented for the formation of extremely weak ion pairs between Na+ and HCO−3 and between Cs+ and CO2−3.