Thermodynamic stabilities of tetra-n-butylphosphonium acetate (TBP-Ace) semiclathrate hydrates in the presence of methane (CH4), carbon dioxide (CO2), nitrogen (N-2), or ethane (C2H6) were measured in a pressure range up to approximately 5 MPa. The dissociation temperature of TBP-Ace + CH4, TBP-Ace + CO2, and TBP-Ace + N-2 semiclathrate hydrates increased drastically with an increase in pressure, which means that CH4, CO2, and N-2 molecules occupy the vacant cages of the TBP-Ace semiclathrate hydrate. On the other hand, the C2H6 molecules hardly occupied the cages, resulting in small pressure dependence of the dissociation temperature. Raman spectra and powder X-ray diffraction patterns of TBP-Ace + CO2 semiclathrate hydrate reveal that the phase transition occurs at 1.04 +/- 0.04 MPa and 285.88 +/- 0.05 K. One of the possible reasons why the phase transition occurs is that the carbonate and/or hydrogen carbonate anions derived from the CO2 molecules are replaced with some of acetate anions in the TBP-Ace + CO2 semiclathrate hydrate. (C) 2019 Elsevier B.V. All rights reserved.