Electrochemical systems are ubiquitous technologies found in water treatment, chemical manufacturing, and energy storage and conversion. Ion-exchange membrane separators reside at the heart of these systems and their ionic conductivity and selectivity dictate the electrochemical cells’ energy efficiency. Because of their polymeric composition, the molecular architectures of ion-exchange membranes are imprecise and contain numerous structural defects. As a result, it has been difficult to carefully probe the underlying physics that governs ion transport and selectivity within these membrane separators. This project investigates counterion condensation phenomena in ion-exchange membranes and correlates it to bulk ionic charge transport and selectivity in precisely defined and long-range ordered materials afforded through the principles of directed self-assembly. The central premise of the work is that condensed counterions in the membrane conduct slower under applied electric fields and the condensed ions aid unwanted co-ion adsorption compromising the selectivity of the membrane. A multi-faceted approach spanning i.) advanced metrology (e.g., 2D atomic force microscopy and x-ray scattering), and ii.) molecular simulations (classical molecular dynamics and quantum dynamics that bridge multiple time and length scales) are employed to correlate the extent of counterion condensation to ionic conductivity and selectivity. After connecting the extent of the condensation phenomena to ion conducting polymers’ charge density and periodic spacing, new macromolecular chemistries will be employed to manipulate the ion pairing interactions so more facile and selective conduction rates can be attained. The implications of the research will lead to a new paradigm in the rational design of polymeric ion-exchange membrane separators that aid high current density for electrochemical systems and subsequently better energy efficiency.