The interfacial and bulk properties of the aqueous solutions of imidazolium-based ionic liquids with different halide anions ([C(12)mim]Cl, [C(12)mim]Br, [C(12)mim]I) were investigated by surface tension and electrical conductivity measurements. The lowest surface tension (gamma(cmc)) and maximum surface excess concentration (Gamma(max)) values from the surface tension measurements showed that [C(12)mim]I had the highest surface activity. The thermodynamic pontentials of micellization (Delta G(m)(0), Delta H-m(0), Delta S-m(0)) indicated that the micellization of [C(12)mim]Cl and [C(12)mim]Br was entropy-driven, while that of [C(12)mim]I was enthalpy-driven at 15 degrees C but entropy-driven above 20 degrees C. This distinct behavior for [C(12)mim]I was thought to be due to the higher binding affinity of I- to the micellar interface.