Solubility data of CO2 and vapor pressure of water in 3.04−4.28 kmol/m3 aqueous N-methyldiethanolamine (MDEA) solutions were obtained at temperatures ranging from 40 to 100 °C and CO2 partial pressures ranging from 0.876 to 1013 kPa. A thermodynamic model was proposed and used for predicting CO2 solubility and water vapor pressure. An enthalpy change of absorption of CO2 in 4.28 kmol/m3 MDEA solution was estimated. The effect of piperazine (PZ) concentration on CO2 loading in MDEA solutions was determined at piperazine concentration ranging from 0 to 0.515 kmol/m3. The results show that piperazine is beneficial to the CO2 loading. The equilibrium partial pressure of piperazine in the PZ−MDEA−H2O system was measured in an Ellis Cell. Results show that the PZ−MDEA−H2O system is a typical negative deviation system, with the strength of deviation decreasing with MDEA solutions.