The impedance method was used to study the charge-transfer interaction between the polymer conducting chain (emeraldine form of polyaniline, PANI) and active molecules in solution (tetrafluorohydroquinone, 4F-Hqui). Equivalent electrical circuits were found for steady-state potentials of the PANI electrode and their parameters were analyzed as dependent on pH. In the presence of 4F-Hqui in the pH range -0.3 to 3, active molecules appear in the double electrical layer and alter the charge exchange between electrode and solution (extrinsic reaction) and faradaic charging of PANI (intrinsic process). Charge-transfer (CT) resistance increases with respect to acid-only solutions; nevertheless, conductivity through the polymer still limits electrical transport at low frequencies. In low pH solutions the protonated imine centers of PANI interact with hydroquinone molecules forming H bonds along the polymer chain. We have proposed that such a structure could serve as an electron path in the moment of the charge transfer. When pH is increased above seven, the CT complex decomposes irreversibly, the polymer electrode is modified and loses its conductivity. We have proposed and discussed possible chemical structures for the modified polymer, its acid transformation, and electrochemical doping.