Ionic insertion or removal from a poly (N-ethylcarbazole) (PECBZ) film modified electrode upon doping/dedoping in perchloric acid was studied by the mirage effect and quartz crystal microbalance (QCMB) techniques. The experimental mirage signal while cycling the potential was compared to the calculated signal based on an exclusive anion exchange limited by diffusion. The mass-charge relation from the QCMB was either obtained during cycling voltammetry or from the mass-charge transfer function measured with a low-level signal. Upon anodic scan, the mass increase is delayed with respect to the charge increase and the measured mirage signal is lower than the calculated one. Upon cathodic scan, the mass-charge relation is linear with a slope of 120 g mol−1 faraday−1, the measured and calculated mirage signals being identical. It is proposed that, starting from a cathodic potential, a conductor-insulator boundary moves from the metal to the solution. A large expulsion of protons and a weak insertion of counter-ions result from a transport control by electric migration in the insulator layer where the transference number of protons is much larger than that of anions. During the cathodic scan, the side of the film against the solution remains conducting and anions are easily exchanged with the solution.