In this work, we show new phase-equilibrium measurements between carbon dioxide and a mixed solvent composed of water, diethanolamine, and methanol. This work is motivated by the ever-stricter natural gas specifications and by the resulting need to develop new solutions to respond to this challenge. Original experimental equipment is developed for this purpose. Its working conditions are limited between 323.15 and 423.15 K and up to a pressure of 10 MPa. Gas solubilities are calculated by the synthetic method based on assumptions including the mass balance equations. The equipment is validated with well-known results obtained on the water + diethanolamine + carbon dioxide system. Two solvent compositions have been investigated: 40/40/20 and 40/20/40 wt % of, respectively, water, diethanolamine, and methanol. The temperature range is similar to 323.15 to 393.15 K with a pressure up to 3.6 MPa. The measurements are correlated by the Kent and Eisenberg approach.