The critical micelle concentrations (CMCs) of sodium carboxylates bearing one or two double bonds near the polar head are determined at various temperatures using cryoscopy, conductimetry, and vapor pressure osmometry techniques. CMC increases with temperature in the range 25 to 40 degrees C. The enthalpy for micellization of monoenic and dienic compounds is determined above 25 degrees C and found to be negative. The exothermic micellization process is attributed to the rigidity of the polar head group which inhibits molecular interactions in the micellar state. CMC increases with increasing number of double bonds. The free energy of micellization per unsaturated CH group (Delta G(m)(CH)) is then determined. The values obtained, which are -1.83, -1.56, -1.27, and -1.18 kJ.mol(-1) at 0, 25, 33, and 40 degrees C, respectively, are higher than those of CH2 group, i.e., -2.53 +/- 0.15 kJ.mol(-1) in the same temperature range. The unsaturated CH group has thus a less hydrophobic character. Moreover the Delta G(m)(CH) value of the compounds studied here is found to be lower than the Delta G(m)(CH) value of end-tail double-bond surfactants. Based on these results, it is concluded that the closer the double bond to the polar head, the higher the hydrophobic character.