Sulfonated poly(aryl ether ether ketone), S-PEEK, is blended with non-sulfonated poly(ether sulfone) (PES) to adjust the properties of ion permeable and ion selective membranes. In this study, membranes are prepared from blends with (i) a S-PEEK content between 10 and 100 wt.% using one S-PEEK batch with a fixed degree of sulfonation and (ii) from batches of S-PEEK with a different degree of sulfonation, but with a fixed S-PEEK content in the blend. The transparent membranes are permeable for ions with selective transport of cations over anions. At contents of S-PEEK below 40%, phenomena related to a percolation threshold of the ion exchange functionalities are observed; the measured ion exchange capacity (IEC) indicates that not all functional groups are accessible in these blends. The transport properties of membranes with a S-PEEK content in the range of 50¿80 wt.% are comparable to those known for commercial ion exchange membranes. In this range, a trade-off between resistance and selectivity with increasing IEC is observed. Both, the ion conductivity and the co-ion transport number increase with increasing IEC. This is mainly caused by the increased water content with increased IEC and the number of water molecules per fixed charge.