The thermodynamic properties of the cationic surfactant cetyltrimethylammonium bromide (CTAB)/water system were investigated using differential scanning calorimetry (235–395 K) and adiabatic calorimetry (78–320 K), and the stable phase diagram was established, in which the locations of phase boundaries were precisely determined. The micellar phase and the lyotropic liquid crystalline (LC) phases were found to transform into a metastable gel phase via rapid cooling, and a metastable phase diagram was also constructed. The hexagonal LC phase was found to consist of two types of phases with different thermodynamic characteristics. The transitions from the separated two-phase “CTAB crystal + solution” to the micellar and LC phases were also investigated, and it was found that the transition occurs in a two-step manner, wherein the transitions at lower temperatures depended on the thermal history. The enthalpy of fusion of the ice that coexisted with the CTAB crystal did not provide any evidence for the CTAB crystal being hydrated by non-freezing water.