Polyelectrolyte membranes (PEM) such as Nafion (DuPont TM) and sulfonated polystyrene based block copolymers used as compartment separators in fuel cells and other electrochemical applications, are also of interest as permselective diffusion barriers in protective fabrics. By means of molecular dynamics (MD) simulations, we explore the molecular mechanisms of sorption and mobility of water and nerve agent simulant dimethylmethylphosphonate (DMMP) in Nafion and sulfonated PS. It is shown that the composition of the solvent strongly affects the membrane permeability, which cannot be predicted purely from the data for pure compounds. This effect is related to complex microphase segregation in the membranes swollen in aqueous solutions of polar solvents. The results obtained demonstrate possible mechanisms of the diffusion of alcohols and alkylphosphonates in hydrated ionomer membranes and reveal properties controlling the utilization of PEM for protective fabrics.