Three-phase equilibrium (pressure-temperature) relation of hydrate+aqueous+fluid phases for the hydrogen (H(2))+tetra-n-butyl phosphonium bromide (TBPB)+water ternary system was investigated in a temperature range of 281.90-295.94 K and a pressure range up to 170 MPa. The behavior of the three-phase coexisting curve indicates no structural transition in the present experimental region. The Raman spectra obtained in the H(2)+TBPB mixed semi-clathrate hydrate crystal reveal that H(2) molecule occupies only small cage compartmentally and the TBPB molecule is encaged with a set of other large cages. (C) 2011 Elsevier Ltd. All rights reserved.