A water-soluble, self-doped, electroactive polymer, poly [3,6-(carbaz-9-yl)propanesulfonate] (PCPS), has been prepared for the first time. Electrochemical ~.,alyses show that this polymer has two separate oxidation states involving interactions of the polymer backbone with the covalently bound, self-doping ions and counterions from electrolyte, indicating that the polymer has the ability to act as a potential-dependent charge-controllable membrane with both cation-exchange and anion-exchange properties. We have used the quartz crystal microbalance technique to investigate these two processes, to verify the self-doping mechanisms exhibited by PCPS, and detail the charge/mass transport relationships in this polymer.