A thermodynamic study on the micellization process of two series of cationic surfactants, viz. the O-dodecyl-N,N'-diisopropylisourea hydrochloride, hydrobromide, and hydroiodide and the O-tridecafluorooctyl-N,N'-diisopropylisourea hydrochloride, and hydrobromide, is reported. In order to explain the effect of the counter ion nature in the micellization process, thermodynamic parameters to include the standard free energy (Delta G degrees(mic)), enthalpy (Delta H degrees(mic)), entropy (Delta S degrees(mic)), and heat capacity (Delta C degrees(p.mic)) of micellization have been discussed. These parameters are evaluated from the variation of the critical micelle concentration (CMC) with temperature by fitting these values to expressions derived on the basis of a micellization thermodynamic model. The heat capacities of micellization (Delta C degrees(p.mic)) were determined from the temperature dependence of Delta H degrees(mic). For both surfactant series, the free energy of micellization is weakly temperature dependent and becomes more negative with increasing the size of the counter ion, indicating a more favored micellization. The enthalpy and entropy of micellization decrease strongly with increasing temperature. The heat capacities of micellization (Delta C degrees(p.mic)) were found to be negative. The enthalpy-entropy compensation phenomenon was observed for all the surfactants studied. (C) 2011 Elsevier Ltd. All rights reserved.