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Knowledge of Micellar behavior of surfactants in aqueous and in co-solvent system is essential for wide range of applications. Association behavior of an anionic surfactant, Sodium dodecylbenzenesulphonate (SDBS) has been studied conductometrically at different temperatures in aqueous and in various compositions of ethylene glycol (EG) and water. The critical micelle concentration (CMC) and degree of counter-ion dissociation values (α) of SDBS increase as the mole fraction of ethylene glycol (EG) increase at constant temperature and also increase with increasing temperature at constant mixture composition. Phase separation model, which gives quantitative theoretical interpretation of thermodynamic parameters of micellization to evaluate the free energy, enthalpy and entropy of micellization over the temperature range of 298-318K. The thermodynamic analysis shows that, although the micellization is less favorable in mixed solvent compared to pure water but the process is spontaneous and exothermic. It was found that the free energy of micellization values become less negative whereas the enthalpy values becomes more negative in binary solvent systems than aqueous solution. The ΔS o m values for aqueous solution of SDBS are higher than that of binary solvent systems that micellization is favoured by entropy gain in aqueous solution.