Isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC) measurements were carried out on various binary mixtures of the nonionic surfactant hexaethylene glycol mono-n-dodecyl ether (C12EO6) and the triblock copolymer "Pluronic" F127 of chemical composition EO97PO69EO97, which is a polymeric nonionic surfactant (EO represents the poly(ethylene oxide) blocks and PO the polypropylene oxide blocks). In all cases mixed micellar aggregates were formed and critical micellar concentrations (cmc's) of binary mixtures containing different mole fractions of the surfactants were measured using ITC. The interaction between the two surfactants shows synergistic behavior. The results were analyzed using a regular solution theory, which introduces a special parameter to characterize the interaction between two surfactant species in a mixed micelle. The cmc's of the F127-rich micelles were also characterized with DSC.