The quantification of gaseous carbon dioxide, CO2, adsorbed by tertiary alkanolamines was performed using a quartz crystal microbalance. Carbon dioxide was injected over piezoelectric quartz crystals coated with different amounts of N,N,N prime ,N prime -tetrakis(2-hydroxyethyl)ethylenediamine (THEED), N,N,N prime ,N prime -tetrakis(2-hydroxypropyl)ethylenediamine (Quadrol), and triethanolamine (TEA), and the frequency decrease of the crystals was recorded. The nature of the interaction of the alkanolamines with CO2 was investigated by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR), fast atom bombardment mass spectrometry (FABMS), and mass spectrometry/mass spectrometry (MS/MS).