The standard industrial process for the purification of natural gas is to remove acid gases, mainly hydrogen sulfide and carbon dioxide, by the absorption and reaction of these gases with alkanolamines, but the lack of reliable and accurate vapor-liquid equilibrium (VLE) data impedes the commercial application of more efficient alkanolamine systems. A novel Fourier-transform infrared (FTIR) technique was developed to make in-situ VLE measurements of acid-gas-aqueous alkanolamine systems and to improve the accuracy of VLE measurements at low hydrogen sulfide and carbon dioxide concentrations. VLE measurements of low carbon dioxide and hydrogen sulfide concentrations in aqueous mixtures of methyldiethanolamine (MDEA) are reported using the new FTIR technique.