The establishment and effects of the Donnan potential, ED, at the polymer/external solution interface in Electrochemically Active Macromolecular films, for the particular case of Polyaniline (Pani) is considered both from the fundamental and experimental viewpoints. In particular, it is demonstrated that E-D should be considered together with the osmotic equilibrium and only under special circumstances, i.e. the osmotic pressure is negligible, ED may be calculated with the classical expression that arises from considering the ionic equilibrium alone. Also, it is demonstrated that, in Pani, the amount of protonated fixed sites depends on the external pH and the Donnan potential. The electrochemical response of the Pani film/aqueous solution system is studied as a function of the pH and the ionic strength. The parameter used in this work to evaluate the influence of ED in this kind of systems is the experimental peak potential, E-p,E-exp, which is the potential at the maximum current in the voltammetric response. Based on previous models, it is obtained an equation for the Ep,exp that allows to interpret the dependence of this parameter on the pH and the ionic strength. (C) 2016 Published by Elsevier B.V.