This work investigated the phase behaviour of tetra-n-butyl ammonium bromide (TBAB) semi-clathrates formed with CO2, and (CO2 + CH4) gas mixture using high-pressure differential scanning calorimetry (HP DSC). The phase equilibrium data of TBAB semi-clathrate formed with (40% CO2 + 60% CH4) in mole fraction were obtained at four TBAB molar concentrations (0.29%, 0.62%, 1.38%, and 2.57%). It was found that (TBAB + CO2 + CH4) mixed hydrate formed at 2.57% TBAB was more stable than that formed at 0.29%, 0.62%, and 1.38% TBAB, and two types of TBAB semi-clathrate (type A and type B) formed at the same time in the presence and absence of the (CO2 + CH4) gas mixture. The dissociation enthalpy of TBAB hydrate formed at 0.62% TBAB was higher than those obtained at 0.29%, 1.38%, and 2.57% TBAB. Besides, the phenomenon of liquid CO2 evaporation was confirmed in the (TBAB + CO2) hydrate formation system with the pressure varying from 3 MPa to 5 MPa. The endothermic heat of liquid CO2 evaporation was found to be much larger than that for the dissociation of (TBAB + CO2) semi-clathrate hydrate. (C) 2019 Elsevier Ltd.