The interfacial and bulk properties of the aqueous solutions of imidazolium-based ionic liquids with different halide anions ([C₁₂mim]Cl, [C₁₂mim]Br, [C₁₂mim]I) were investigated by surface tension and electrical conductivity measurements. The lowest surface tension (γ_cmc) and maximum surface excess concentration (Γ_max) values from the surface tension measurements showed that [C₁₂mim]I had the highest surface activity. The thermodynamic pontentials of micellization (ΔG⁰_m, ΔH⁰_m, ΔS⁰_m) indicated that the micellization of [C₁₂mim]Cl and [C₁₂mim]Br was entropy-driven, while that of [C₁₂mim]I was enthalpy-driven at 15 °C but entropy-driven above 20 °C. This distinct behavior for [C₁₂mim]I was thought to be due to the higher binding affinity of I^– to the micellar interface.