The results of critical micelle concentration of aerosol OT in pure and binary mixtures of water, formamide, ethylene glycol, and dioxane determined by the methods of conductance, surface tension, spectrophotometry, and calorimetry are presented. The cmc values by the different methods are in agreement with one another and have shown positive deviations with maxima in all the binary solvent mixtures at characteristic compositions. In this behavior, mixtures containing water are distinctly different than those without water. The water-containing mixtures have shown both negative and positive enthalpies of micellization whereas those without water are all associated with positive enthalpy. The entropies of micellization are all positive, and they nicely compensate the enthalpies of the process. The heat capacities of the resulting mixtures have evidenced weak interaction among the molecules of the mixed solvents.