A thermochemical investigation of the series of 1-alkyl-3-methylimidazolium ionic liquids (ILs) with the trifluromethanesulfonate CF3SO3- anion is presented. Absolute vapor pressures and vaporization enthalpies were measured by using a quartz crystal microbalance (QCM). The enthalpies of solutions were determined by solution calorimetry. Gas-phase enthalpies of the formation of ILs were calculated by applying the high-level quantum-chemical method G3MP2. From a combination of experimental and theoretical results, the enthalpy of formation of aqueous CF3SO3- anion was derived for the first time.