The effect Of CO2 on the phase behavior of the reaction system and equilibrium conversion for esterification of acetic acid and ethanol in ionic liquid (1-butyl-3-methylimidazolium hydrogen sulfate, [bmim][HSO4]) was studied at 60.0 degrees C up to 15 MPa. It was demonstrated that there was only one phase in the reaction system in the absence of CO2,. The reaction system underwent two-phase-three-phase-two-phase transitions with increasing pressure. The pressure Of CO2 or the phase behavior of the system affected the equilibrium conversion of the reaction markedly. As the pressure was less than 3.5 MPa, there were two phases in the system, and the equilibrium conversion increased as pressure was increased. In the pressure range of 3.5-9.5 MPa, there existed three phases, and the equilibrium conversion increased more rapidly with increasing pressure. As the pressure was higher than 9.5 MPa, the reaction system entered another two-phase region and the equilibrium conversion was nearly independent of pressure. The total equilibrium conversion was 64% without CO2 and could be as high as 80% as pressure was higher than 9.0 MPa. The apparent equilibrium constants (K-x) in different phases were also determined, showing that the K-x in the middle phase or top phase was much greater than that in the bottom phase.