This paper reports the accurate phase equilibrium measurements of two ionic semiclathrate hydrates with tetrabutylammonium bromide (TBAB) and tetrabutylammonium chloride (TBAC). These ionic semiclathrate hydrates are suggested as cool energy storage media for air-conditioning system since their dissociation heats of phase transitions are as large as 200-500 kJ/kg and they form at 278-293 K under atmospheric pressure. Tetrabutylammonium bromide and tetrabutylammonium chloride form ionic semiclathrate hydrates and there are several previous reports of the equilibrium temperatures of these hydrates in the literature. However, there is inconsistency in the literature data of equilibrium temperatures. Also, there are no clear notifications of experimental procedures and uncertainty of measurements in some of the previous reports. Therefore, we have performed accurate measurements of the phase equilibrium of tetrabutylammonium bromide and tetrabutylammonium chloride hydrates and the comparison with the literature data is also made in this paper. The highest equilibrium temperature for tetrabutylammonium bromide system was 285.9 K at 0.35 < w(TBAB) <0.37, where w(TBAB) denotes the mass fraction of tetrabutylammonium bromide (or the mole fraction of tetrabutylammonium bromide, 0.029 < chi(TBAB) <0.032), under atmospheric pressure. That for tetrabutylammonium chloride system was 288.2 K at w(TBAC) = 0.35, where w(TBAC) is the mass fraction of tetrabutylammonium chloride (or the mole fraction of tetrabutylammonium chloride, x(TBAC) = 0.034), under atmospheric pressure. (C) 2012 Elsevier B.V. All rights reserved.