A method was developed to determine the concentrations of the ionic species present in the CO2−NH3−H2O system. The 13C NMR spectra of these solutions contain two peaks, one for the carbamate and one for the carbonate and bicarbonate. The method uses the area ratio and the position of the latter peak to determine concentrations. Ion concentrations were determined at 17 different concentrations for total NH3 and CO2 and at two different temperatures, 25 and 35 °C. All measurements were for solutions in which the total concentration of NH3 was greater than that of CO2. A comparison of three different thermodynamic models showed that although the models overpredicted the amount of carbonate, all were in reasonable agreement with experiment when the total ammonia concentration was less than 3 mol/L of solvent. At the higher concentrations, all the models badly underpredicted the amount of carbamate formed and overpredicted the amount of carbonate.