Differential scanning calorimetry (DSC) was used to determine the phase diagram of the binary system, trimethyloloethane (TME) + water; a system of interest in relation to latent heat storage organic hydrate materials. A stable hydrate was formed between the two components, The hydrate melted incongruently at 303.0 K with a latent heat of 218 kJ kg(-1). The eutectic point between the hydrate and ice was observed at 27 1.1 K. The composition of the hydrate estimated from the peak area of the eutectic melting was 3 mol of water against 1 mol of TME. Since this composition of the hydrate was different from that reported in literature, a new sample of the hydrate was prepared. After careful drying of the hydrate, gravimetric analysis gave the stoichiometric amount of water to be (3.00 +/- 0.01). From a single crystal X-ray structural analysis, it was found that the hydrate crystal had a layered structure with alternate TME and water layers. Each TME layer was linked by hydrogen bonding through the water layer. In an asymmetric unit there was one TME molecule, and the oxygen atoms of the water molecules were statistically located on five positions with occupancy factor of 1.0, 1.0, 0.5, 0.25, and 0.25, respectively. Therefore, the stoichiometric amount of water against one TME molecule is 3, Hence, the X-ray diffraction confirmed the proposed hydration number of the stable hydrate.