In the present contribution we investigated the influence of the acid pretreatment procedure of Nafion (R) 117 membranes on the resulting ionic conductivity by using electrochemical impedance spectroscopy. For this purpose, a setup allowing to measure the ionic conductivity at different temperatures, relative humidities, and contact pressures was developed. The state of protonation was determined by the ionic exchange capacity which is a measure of the amount of protonated sulfonic acid groups for the overall ionic exchange ability of the Nafion (R) membrane. The state of hydration was determined by water uptake and water vapor adsorption while structural observation was conducted by infrared spectroscopy. Additional thermophysical characterization was performed using differential scanning calorimetry. It was found that the pK(a) value and the concentration of the used acid for the pretreatment mainly influence the ion exchange capacity, whereas the temperature during pretreatment mainly influences the water uptake. The combination of high values for temperature, relative humidity and contact pressure leads to enhanced ionic conductivity of Nafion (R) membranes with a maximum value of 85.2 mS cm(-1) for the membrane pretreated with HCl. (C) 2015 Elsevier B.V. All rights reserved.