The CO, absorption capacities for binary solutions of tetrabutylphosphonium formate ionic liquid (IL) and water are measured at 0.1 MPa of CO, for various compositions (up to 10 in mole ratio of water to the IL) as a function of temperature (-24 to 60 degrees C). The capacities measured as the mole of CO2 with respect to 1 mol of the IL varied in a wide range from 0.01 to 1.0. Capacities decreased monotonously with temperature at a fixed absorbent composition. When the water concentration is varied at a fixed temperature, capacity takes a maximum when the water mole ratio is approximately 1. In contrast to CO2 absorbents made from acetate ILs, the present system loses affinity to CO2 in the absence of water. The equilibrium constant for the chemisorption is defined in two ways by assuming that CO2 is captured as a free bicarbonate ion (HCO3-) or a complex ion with formic acid ([(HCOO) (HCO3)H](-). The van't Hoff plot of these two equilibrium constants are both linear. However, the estimated enthalpy change of absorption is ca. 20 kJ/mol larger for the former scenario than the latter.