This paper reports an experimental study to measure the thermodynamic properties of ionic semiclathrate hydrate formed with tetrabutylphosphonium bromide. The thermodynamic measurements were performed on the temperature-composition phase diagram of the hydrate in the range of the mass fraction of tetrabutylphosphonium bromide w(TBPB) from 0.10 to 0.50 and the hydrate dissociation enthalpy, where w(TBPB) denotes the mass fraction of tetrabutylphosphonium bromide. The highest equilibrium temperature for tetrabutylphosphonium bromide hydrate was 282.4 K at 0.33 <= w(TBPB) <= 0.35. Dissociation enthalpy was measured to be 214 kJ/kg at w(TBPB) = 0.35 with a differential scanning calorimetry. An attempt was made to determine the crystallographic structure of the tetrabutylphosphonium bromide hydrate by single-crystal X-ray diffraction measurements. (C) 2011 Elsevier B.V. All rights reserved.