The thermodynamic equations necessary to obtain the enthalpy of micelle formation for binary mixed surfactant systems from the titration calorimetry of the aqueous solution of surfactant mixture were developed. They were applied to the experimental results of the following mixtures: pentaethylene glycol monodecyl ether(C10E5), pentaethylene glycol monooctyl ether(C8E5), and C1OE5-tetraethylene glycol monooctyl ether(C8E4). It was found that C10E5 and C8E5 molecules are ideally mixed in the micelle from the viewpoint of the enthalpy as well as the Gibbs energy, whereas C10E5 and C8E4 molecules mix in a slightly nonideal fashion due to the positive excess enthalpy. Furthermore, it was shown that the excess entropy of mixed micelle formation of the C10E5-C8E4 system is positive.