This paper reports the vaporliquidhydrate three phase equilibrium conditions of ionic semiclathrate hydrates formed in the system of water + tetra-n-butylammonium bromide pressurized with carbon dioxide. The three phase equilibrium temperatures were measured in the ranges from 278.8 to 287.4 K of temperature and from 0.05 to 0.45 of mass fraction of tetra-n-butylammonium bromide (w(TBAB)). Measurements were performed at the system pressures of 0.3 MPa, 0.6 MPa, and 1 MPa. For the three system pressures, the equilibrium temperatures increased with the increase in w(TBAB) for w(TBAB) < 0.35 and decreased for w(TBAB) > 0.40. The highest equilibrium temperatures were obtained at w(TBAB) = 0.35 and 0.40. The equilibrium temperatures of the tetra-n-butylammonium bromide + carbon dioxide hydrates were higher than those of the simple tetra-n-butylammonium bromide hydrates formed under atmospheric pressure. The improvement of the thermodynamic stability of the tetra-n-butylammonium bromide + carbon dioxide hydrates is ascribed to the incorporation of carbon dioxide molecules into the hydrate cages. The data obtained in the present study may be utilized to specify the thermodynamic conditions to form the tetra-n-butylammonium bromide + carbon dioxide hydrates without forming the simple tetra-n-butylammonium hydrates.