The heat capacities of the binary system consisting of a nonionic surfactant, hexaoxyethylene n-dodecyl ether (C12E6), and water were precisely measured as a function of temperature by adiabatic calorimetry over the temperature and the concentration ranges where lyotropic liquid crystals are formed. The enthalpy and entropy of transitions were determined for all transitions observed. The enthalpy and entropy of transition between liquid crystalline phases suggest that the liquid crystalline phases in this system are mainly constructed by C12E6 molecules with a fixed amount of Water. The excess heat capacities, as estimated by measuring the heat capacity of neat C12E6, are positive over the entire ten perature and concentration ranges. The excess heat capacities also support the suggestion given above concerning the role of C12E6 molecules on the structure building.