The enthalpy of the mixing of water and tetraethylene glycol monooctyl ether (C8E4) was measured precisely as a function of concentration of surfactant and temperature by using the high-accuracy isothermal titration calorimeter. The differential enthalpies of solution of monomer and micelle and the enthalpy of micelle formation were evaluated by applying the thermodynamic relations to the enthalpy of mixing. It was shown that the dissolution of pure liquid C8E4 is exothermic and the micelle formation is endothermic, respectively. It was also shown that the micellization process is exothermic from the viewpoint of the partial molar enthalpy of water. The heat capacity changes accompanied by the dissolution and micellization processes were calculated. The partial molar heat capacity change vs molality curves for C8E4 and water were found to have a sharp maximum or minimum, and both its height and width are strongly temperature-dependent.