Binary surfactant mixtures of Tween 85/Triton X100 and Tween 85/Triton X165 have been investigated in water solution over the temperature range from 273.15 K to 323.15 K. At the temperatures of 273.15 K and 283.15 K between different building units of binary mixed micelles, synergistic interactions exist whose size is not affected by difference in length of the polyoxyethylene chains. At higher temperatures, the longer polyoxyethylene chain of Triton X165 shows a more stabilizing effect in mixed micelle with Tween 85 than in mixed micelles of Tween 85/Triton X100. For the investigated binary mixed micelles, regular solution theory approximation about zero value of excess entropy of binary mixed micelle formation cannot be accepted. (C) 2016 Elsevier Ltd.