The thermodynamic stability and hydrogen occupancy for the hydrogen+trimethylamine mixed semiclathrate hydrate system were investigated by means of phase equilibrium (pressure-temperature) measurements and Raman spectroscopic analyses. The hydrogen molecule gradually advanced to occupy the empty small cage of trimethylamine hydrate in proportion to pressure increase. Almost ail small cages were filled up with the hydrogen molecules at about 80 MPa. Isothermal Raman spectroscopic analysis showed that the absorption-rate of hydrogen to the pre-treated trimethylamine hydrate was comparable to those of tetrahydrofuran hydrate. Only one hydrogen molecule was enclosed with one small cage at the equilibrium state in pre-treated trimethylamine hydrate. (C) 2009 Elsevier Ltd. All rights reserved.