Two types of sulfonic acid membranes perfluorinated (Nafion) and poly(ether ether-ketone) (SPEEK) were modified by impregnation with the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate (BMI center dot BF4). The ionic exchange between the butyl-methylimidazolium (BMI) cation and the sulfonic acid groups of the polymers are limited by diffusion into the membrane pores and can be controlled via the time of contact with the IL solution. Infrared (IR) and thermogravimetric (TGA) analyses confirmed the incorporation of the BMI+ cations into the polymers. The polymer electrolyte membranes modified with ILs exhibit enhanced conductivity at a relative humidity of 70% and at elevated temperatures despite their reduced absorptive capacity. (C) 2014 Elsevier B.V. All rights reserved.