The heats of mixing of gaseous CO2/CH4 mixtures with aqueous solutions of methyldiethanolamine and diethanolamine have been determined using an isothermal flow calorimeter. Data were collected at 26.7, 76.7 and 126.7 degrees C, at total pressures of 1.38, 3.45 and 6.90 MPa, with aqueous amine concentrations of 20, 35 and 50 wt%. The nominal partial pressure of CO2 in the gaseous feed stream was 0.069 MPa at a total pressure of 1.38 and 0.69 MPa at 3.45 and 6.90 MPa total pressure. A model developed to predict thermodynamic properties of these systems includes reaction equilibria, dissolution of methane, vapor-phase non-idealities, vaporization of water and amines in addition to the acid gas absorption.