Nafion membranes were modified by incorporation of ionic liquid (IL) cations, at controllable degrees. These modified membranes were then characterised with the use of different techniques. X-ray photoelectron spectroscopy (XPS) demonstrated that the IL cations are incorporated in the membrane material and allowed to evaluate the distribution of the cation in the membrane surface layer. Electrochemical impedance spectroscopy (EIS)was used to monitor membrane modification by incorporation of IL cations, and to study the electrical properties of the modified membranes, with different degrees of cation incorporation. These results were adjusted to mathematical models of equivalent electrical circuits to extract data of membrane ionic conductivity. Finally, thermogravimetry studies allowed assessing the stability of the modified membranes with increasing temperature in terms of their water content, and these results were used to establish a possible relation between the membrane stability at high temperatures with their water content and degree of structuring. It was concluded that it is possible to obtain membranes with tailored properties depending on the type of IL cation and its degree of incorporation. (C) 2009 Elsevier B.V. All rights reserved.