The micellization of six cationic gemini surfactants with various counterions, [C12H25(CH3)(2)N(CH2)(6)N-(CH3)(2)C12H25]X-2, designated as C12C6C12X2, with X = F-, Cl-, Br-, Ac-, NO3-, and (1)/2SO42- in aqueous solutions has been investigated by isothermal titration microcalorimetry (ITC) and conductivity measurements. The interaction of these surfactants with DNA in aqueous solutions has also been investigated by isothermal titration microcalorimetry. The critical micelle concentration (CMC) and the degree of micellar ionization (alpha), the critical aggregation concentration (CAC), the saturation concentration (C-2) of the aggregation, and the associated thermodynamic parameters were determined. The nature of the counterion significantly affects the processes of both micellization and aggregation. The trends for aggregation basically follow the Hofmeister (lyotropic) series, but the pattern of the variation of the enthalpy of aggregation often revealed a more complex behavior. Among the counterions examined, SO42- is the most effective anion for decreasing the CMC (or CAC). Both aggregation processes are mainly entropy-driven since the values of the entropy changes multiplied by temperature are much larger than the absolute values of the enthalpy changes. The binding of micelles to DNA is strongly dominated by the positive entropy gain on release of the small counterions from the micelles and from DNA. The interaction of all of the surfactants with DNA was dependent on the DNA concentration and may be associated with each micelle interacting with more than one DNA molecule.