The values of the CMC of TX-100 as well as the TX-100 and CTAB mixture in the water + ethanol solution were determined from the isotherms of the surface tension (Figs. 3, S6–S8 and Tables S9–S12),density (Fig. S9), dynamic viscosity (Fig. S10) and for the TX-100 and CTAB mixture also from the specific conductivity (Fig. 4). The values of the CTAB CMC were taken from our previous paper [31]. Independently of the method used for the CMC determination, its values decrease slightly to the minimum (Figs. 5 and 6), according to the suggestions of some investigators dealing with the aqueous solutions of single surfactants at low ethanol concentration[32–35]. This minimum appears at different ethanol concentrations depending on the composition of the TX-100 and CTAB mixture.It is known that there are two decisive effects in the process of surfactant micellization, and these are enthalpy and entropy. The decrease of enthalpy and the increase of entropy cause the CMC decrease. We have shown earlier that the surfactant mixture practically does not affect the apparent and partial volume of ethanol and water. It means that the interactions between the ethanol molecules and those of water are stronger than between surfactants and water or surfactants and ethanol. However, it is possible that water molecules bonded to the oxyethylene groups of TX-100 and the hydrophilic group of CTAB can be removed by ethanol ones and then the decrease of the repulsive forces between the hydrophilic groups of surfactants occurs and the tendency of the surfactants to form micelles increases. This process can be connected with the decrease of the enthalpy in the micellization process and the small changes of the entropy effect. However, at the higher ethanol concentration, the decrease of the entropy effect on the micellization of surfactants takes place and therefore the CMC increase is observed.At the ethanol concentration close to its CAC, the micelle formation was not detected by any method used for the CMC determination Of course, it is in accordance with the suggestions of Zana [36] but contrary to the conclusions drawn by Nazir et al. [35] from their studies dealing with single surfactants.