In this work, the monoalkylcarbonate ((N-hydroxyethyl) (N-methyl) (2-aminoethyl) hydrogen carbonate) formation in the system methyldiethanolamine (MDEA) water (H2O)- carbon dioxide (CO2) is investigated by nuclear magnetic resonance (NMR) spectroscopy. Aqueous solutions containing 0.4 g/g of MDEA were loaded with CO2 in valved NMR tubes, and the composition of the liquid phase in equilibrium was determined in situ at 298 K at pressures up to 11 bar. By two-dimensional NMR, the presence of monoalkylcarbonate was verified, which has been widely overlooked in the literature so far. The experimental data of this work and reevaluated NMR data obtained in previous work of our group were used to calculate chemical equilibrium constants of the proposed monoalkylcarbonate formation. A model taken from the literature that describes the solubility of CO2 in aqueous solution of MDEA and the corresponding species distribution is extended so that it can account for the monoalkylcarbonate in the liquid phase as well. The extended model is validated using NMR. data in the temperature range 273-333 K. The study shows that more than 10 mol % of the absorbed CO2 is bound as monoalkylcarbonate under conditions relevant for technical applications.