Apparent molar relative enthalpies were measured for the nonionic ethoxylated surfactant CH3-(CH2)(5)-(OCH2-CH2)(5)OH (C6E5) in aqueous solution at constant molality of the ionic surfactant CH3-(CH2)(5)-SO3-Na+ (C6SNa) at 25degreesC. The experimental data obtained by a stepwise dilution process allowed evaluation of the C6E5 first interaction parameter at several constant molalities Of C6SNa. The C6E5 critical micelle composition as a function of the C6SNa molality was also estimated. The experimental calorimetric data, together with the mixed micelles composition computed in the past by some of us [Ciccarelli et al., Langmuir 14, 7130 (1998)], allowed computation of the Deltah(Mic) of micellization. The experimental data are compared to those predicted by the ideal solution model and regular solution model of mixed micellization. From a calorimetric study performed on the water-hexanol-C6SNa and water-penthaethylene glycol-C6SNa model systems, it can be argued that the interactions among the hydrophilic heads in the C6E5-C6SNa mixed micelles prevail on the contribution of the hydrophobic tails in ruling the enthalpic properties of the system.