Heterogeneous sulfonation was successfully utilized to convert the surfaces of PC and PI track etched membranes with 20 nm pores into ion exchange membranes. Short sulfonation times of 1-5 min using gaseous sulfuric trioxide were sufficient to achieve good coverage of ion exchange sites on the membrane surface yet minimal structural damage to the bulk material. Consequently, the ion exchange capacities were relatively low, up to 0.015 meq/g, compared to literature. Electroselectivity typical of ion exchange materials was observed for these membranes and metal uptake of higher valences was preferred Eu3+ > Co2+ > Cs+. Clear differences in metal uptake properties were measured between the sulfonated PC and PI membranes with different ions. As a consequence of relatively high metal uptake and certain experiment set up also Donnan exclusion was determined to play a role in the overall ion exchange process. (C) 2011 Elsevier Ltd. All rights reserved.