This work addresses the experimental measurements of the surface tension of eight imidazolium based ionic liquids (lLs) and their dependence with the temperature (288-353 K) and water content. The set of selected ionic liquids was chosen to provide a comprehensive Study of the influence of the cation alkyl chain length, the number of cation substitutions and the anion on the properties under study. The influence of water content in the surface tension was studied for several lLs as a function of the temperature as well as a function of water mole fraction, for the most hydrophobic IL investigated, [omim][PF6], and one of the more hygroscopic IL, [bmim][PF6]. The surface thermodynamic functions such as surface entropy and enthalpy were derived from the temperature dependence of the Surface tension values. (c) 2007 Elsevier Inc. All rights reserved.