The micellization properties of cationic symmetric gemini surfactants, [CmH2m+1(CH3)(2)N(CH2)(6)N-(CH3)(2)CmH2m+1]Br-2 (designated as CmC6CmBr2, with m = 7, 8, 9, 10, 11, 12, and 16), has been investigated by isothermal titration microcalorimetry. The critical micelle concentration (CMC) and enthalpy of micellization (Delta H-mic) were determined from calorimetric titration curves. The linear decreasing of log CMC with increasing the length of the hydrophobic chain is consistent with an increase in the hydrophobicity of the alkyl chain. Interestingly, with increasing the length of the alkyl chain, the Delta H-mic values of the surfactants with even numbered alkyl chains vary from endothermic to exothermic, whereas the Delta H-mic, values of the surfactants with odd numbered alkyl chains are all endothermic and tend to become more endothermic. The pronounced even/odd effect in Delta H-mic is discussed with respect to the "donor-acceptor" interaction.