Self-association of three long-chain imidazolium ionic liquids (ILs), [C(12)mim]Br, [C(14)mim]Br, and [C(16)mim]Br, in aqueous solution was studied by surface tension measurements over a temperature range From (278.15 to 328.15) K. Effects of temperature and hydrocarbon chain length of the three long-chain ILs oil the critical micelle concentration (CMC) were examined. Thermodynamic parameters. Delta(mic)G, Delta(mic)H, and Delta(mic)S, of micellization were determined by applying a mass-action model equation. Isothermal titration microcalorimetry was used to obtain the enthalpy change upon micellization of the three long-chain ILs at 298.15 K. Moreover, the CMCs and the thermodynamic parameters (Delta(mic)G, Delta(mic)H, and Delta(mic)S) were determined based on the isothermal titration microcalorimetry results. These CMC values are approximately equal to file CMCs obtained by surface tension measurement.