To develop the "hydrate thermal energy storage technology", it is desired to reveal the mechanism of the thermophysical property manifestation on ionic semiclathrate hydrate. The influence of valence on thermophysical properties has not been investigated so far. In the present study, the phase equilibrium temperatures and the dissociation heats of tetrabutylphosphonium sulfate ((TBP)(2)SO4) ionic semiclathrate hydrate were experimentally studied. (TBP)(2)SO4 hydrate consists of the bivalent anion, SO42. The highest phase equilibrium temperature was 6.5 degrees C at the mass fraction range from 0.363 to 0.384. The largest dissociation heat was (168.8 +/- 2.4) kJ.kg(-)(1) at the mass fraction 0.374. These values of the phase equilibrium temperature and the dissociation heat were respectively lower and smaller than those of the ionic semiclathrate hydrates with the monovalent anion. As a new choice for thermal energy storage medium, (TBP)(2)SO4 hydrate would be suitable to the automotive air conditioner and the cold chain for blood or perishable foods. (C) 2018 Elsevier Ltd.