In this study the effects of cation exchange polymer coatings on activated carbon electrodes for capacitive deionization (CDI) were investigated. Electrodes were fabricated from activated carbon, graphite and PVDF, then coated with sulfonated poly(arylene ether sulfone) random copolymers. Additional resistance was created by the coating layer, however compared with the uncoated electrodes the coatings did not significantly affect the rate of charge build up. Electrochemical impedance spectroscopy (EIS) results indicated that both capacitance and charging resistance are influenced by polymer conductivity, water uptake and the thickness of the coating layer. The results also indicated that in addition to functioning as a superficial charge barrier, copolymer that penetrates into the carbon substrate might offset the loss in capacitance caused by PVDF binder pore blockage. © 2014 Elsevier B.V.