Management for the temperature increase of lithium-ion batteries in electric vehicles (EV) is required for safety and energy efficiency. As a cooling medium for lithium-ion batteries, one suitable candidate is trimethylolethane (TME) hydrate, which may be utilized as a phase change material (PCM). This hydrate has a large dissociation heat and will form/dissociate at approximately 30 °C under atmospheric pressure conditions. A cooling system which uses TME hydrate can have a high performance with regards to the efficiency and the stability of heat transfer process. However, in previous studies, the equilibrium temperature and the dissociation heat of TME hydrate have been reported only for a TME mass fraction of 0.625. Since TME hydrate forms in an aqueous solution, it is necessary to know how these thermophysical properties change for a range of TME mass fractions. Therefore, in this study, the equilibrium temperatures and the dissociation heats were measured respectively by the visual observation method and differential scanning calorimetry measurements for TME mass fraction range from 0.20 to 0.80. The highest equilibrium temperature was 29.6 °C at the mass fraction of 0.60. For the cooling system, the largest dissociation heat was 190.1 kJ kg−1, also at the mass fraction of 0.60.