The possibility of hydrogen storage in mixed semi-clathrate hydrates formed from hydrogen + tetrabutylammonium hydroxide (TBAOH)+ water was studied using high pressure differential scanning calorimetry (DSC) and isochoric reactor experiments (p-V-T). The phase diagram of the system TBAOH + water was first investigated, confirming the existence of one congruent melting hydrate and one non-congruent melting hydrate. Secondly, the effect of hydrogen pressure was studied under the range from 0 to 40 MPa, and salt mole fractions from 0.0083 to 0.0244. (p, T) equilibrium data for hydrate dissociation in the system H-2 + TBAOH + water were determined. These results showed that hydrogen has a strong stabilizing effect on the non-congruent TBAOH hydrate structure, dissociation temperatures being increased by about 8 K under a pressure of 40 MPa. This effect was attributed to hydrogen enclathration into the TBAOH hydrate structure. Hydrogen pressure revealed to have much less effect on the stability of the congruent melting TBAOH hydrate, the increase in dissociation temperatures being of the order of 1 K at 40 MPa, therefore indicating an absence of H-2 enclathration. A volumetric measurement of hydrogen storage capacity of hydrate was performed. The amount of gas entrapped in the hydrate phase was measured to be 0.35-0.47 wt% over the pressure range 10-20 MPa. (C) 2013 Elsevier B.V. All rights reserved.