The micellization behavior of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in mixed solvent media of polyethylene glycol 200 (PEG 200) and water with varying mass fractions of PEG 200 has been studied by conductivity and isothermal titration microcalorimetry (ITC) methods. The results showed that micelles existed at a certain ratio of PEG 200 in the mixed solvent. The critical micelle concentration (CMC) of AOT increased with increasing PEG 200 amount in mixed solvent, with the extent of the increase dependent on the decreased polarity and "structure-breaking effect" of the cosolvent. From the change in the Gibbs free energy of micellization (Delta G(m)degrees) relative to pure water, it was found that the systems containing a lower amount of PEG 200 (<60 wt %) had a more negative value of Delta G(m)degrees. This result confirmed that the mixed solvent with low concentration of PEG 200 was favorable for the formation of surfactant micelles, while a higher ratio of PEG 200 was unfavorable for the micellization process. The values of the other two thermodynamic parameters (Delta H-m degrees and -T Delta S-m degrees) indicated that the micelle formation was entirely entropy driven for the systems with pure water or high content of PEG 200 (>= 80 wt %). The other systems were driven by entropic as well as enthalpic contributions. There are complex factors involved in the effect of mixed solvent composition on the energetics of micelle formation. The polarity, dielectric constant, polymer concentration, and position of the added polymer in the micelle may be worth considering for understanding the self-assembly behavior of the surfactant. In addition, the self-assembly behaviors of AOT in ethylene glycol (EG)/water and PEG 400/water were studied and compared. It was found that the variation trend of the parameters showed a similarity, but there are subtle differences between the three systems.