Surface and bulk chemical changes in a Nafion membrane as a result of room temperature ionic liquids (RTILs) incorporation were determined by X‐ray photoelectron spectroscopy (XPS) and elemental analysis, respectively. RTILs with different physicochemical properties were selected. Two imidazolium based RTIL‐cations (1‐octyl‐3‐methylimidazolium and 1‐butyl‐3‐methylimidazolium) were used to detect the effect of cation size on membrane modification, while the effect of the RTIL hydrophilic/hydrophobic character was also considered by choosing different anions. Angle resolved XPS measurements (ARXPS) were carried out varying the angle of analysis between 15° and 75° to get elemental information on the Nafion/RTIL‐modified membranes interactions for a deepness of around 10 nm. Moreover, changes in the RTIL‐modified membranes associated to thermal effect were also considered by analyzing the samples after their heating at 120 °C for 24 h. Agreement between both chemical techniques, bulk and destructive elemental analysis and surface and non‐destructive XPS, were obtained.