The enthalpy of mixing n-octyl penta(oxyethylene glycol)(C₈E₅) with water has been studied at 303.15 K by titration and mixing calorimetry. In the region of the critical micelle concentration (c.m.c.), when liquid surfactant is added to water, a sudden endothermic shift is observed in the differential enthalpy of mixing of C₈E₅. The variation of the differential enthalpy with composition in this region gives information on the average size of the non-ionic micelles. Using the mass-action law model, an aggregation number of 41 was obtained for the micelles. The value is lower than those obtained from recent spectroscopic studies. The thermodynamic quantities of micelle formation of C₈E₅, and of C₈E₄, in water have been obtained. For binary mixtures the enthalpy of mixing was exothermic for all compositions. The apparent and partial molar enthalpies of mixing of the surfactant and the partial molar enthalpies of the water have been calculated. Beyond the c.m.c. the partial molar enthalpies of the two components vary regularly with the composition.