This investigation studied the resulting nanostructure and transport properties of ionic membranes composed of sulfonated copolymers with thermoplastic and elastomeric blocks. Poly(styrene-isobutylene-styrene) (SIBS) of different number average molecular weight (Mn) and polystyrene weight fraction were sulfonated to various levels of ion exchange capacity. The solvent-casted membranes were characterized using several techniques including elemental analysis, Fourier transform infrared spectroscopy-, thermogravimetric analysis, and small-angle X-ray scattering. These techniques were used to evaluate the thermal and physical properties of the membranes, which in turn, allowed for the comparison of resulting morphologies and selectivities. In addition, counter-ion substitution (Mg+2, Ca+2, Ba+2) was used to crosslink the sulfonated polymers to further influence their selectivity. Vapor permeabilities were measured at 308 K using dimethyl methyl-phosphonate, a chemical compound similar to Sarin Gas, and water, to evaluate the selectivity of the membranes and their potential application for chemical and biological protective clothing. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013