Polymer electrolytes, which are commonly used as separator materials in electrochemical devices, have ionic conductivity that is thought to be controlled by segmental mobility. Thus, any improvements made toward increasing ionic mobility come at the expense of mechanical integrity. However, selectively solvating the ionic domain, the region responsible for ion conduction, with water or polar organic solvents presents a potential opportunity to circumvent this physical constraint. Here, we explore the role of hydration on the transport properties of membranes formed from randomly sulfonated polystyrene (PS-r-sPS). We find that the water volume fraction underpins an intrinsic trade-off between separator permselectivity (Psi(m)) and ion conductivity (kappa) - thus, improvements in ion diffusion because of increased water content come at the expense of charge density in the membrane which yields a reduced Psi(m). We provide a summary of the Psi(m)-kappa trade-off for a suite of commercially available separators to elucidate structure-property relationships and present methodologies for improving both Psi(m) and kappa.