Phase equilibrium (pressure-temperature) relations of the hydrogen + tetra-n-butyl ammonium bromide mixed gas hydrate system have been measured for various concentrations of tetra-n-butyl ammonium bromide aqueous solutions. The three-phase equilibrium curves obtained in the present study are shifted to the low-temperature or high-pressure side from that of the stoichiometric tetra-n-butyl ammonium bromide solution. Each three-phase equilibrium curve of hydrogen + tetra-n-butyl ammonium bromide mixed gas hydrate converges at the atmospheric equilibrium point of the pure tetra-n-butyl ammonium bromide hydrate for the mother aqueous solution of same mole fraction. The hydrate-cage occupancy of hydrogen in the hydrogen + tetra-n-butyl ammonium bromide mixed gas hydrate is in variable in a range of tetra-n-butyl ammonium bromide mole fraction from 0.006 to 0.070 in the aqueous solution. Hydrogen is entrapped only in the small cages of tetra-n-butyl ammonium bromide hydrates. (c) 2007 Elsevier Ltd. All rights reserved.