We investigated the phase equilibrium boundary of tri-n-butylphosphine oxide (TBPO) semiclathrate hydrates incorporated with CH 4 , CO 2 , and H 2 . TBPO aqueous solutions with a molality (m) of (1.61 and 1.98) mol·kg −1 were used for hydrate formation, which corresponded to the clathrate structures of TBPO· 34.5H 2 O and 28H 2 O, respectively. The phase boundary at both concentrations was shifted to the promotion region represented by lower pressures and higher temperatures, compared to each simple gas hydrate. In particular, TBPO + CO 2 double hydrate presented mild hydrate stabilization conditions of <1 MPa at (280 to 285) K. Additionally, the dissociation enthalpy (ΔH d ) calculated from the phase boundary curves for the TBPO + CO 2 double hydrates was almost the same as that for tetra-n-butylammonium bromide (TBAB) + CO 2 double hydrate (219.5 kJ·mol −1 for m = 1.61 mol·kg −1 and 211.6 kJ·mol −1 for m = 1.98 mol·kg −1 ). These results demonstrate that the TBPO + CO 2 double hydrate could be used as refrigerants for cold storage and transportation.