The present work is concerned with a thermodynamics model that describes the redox switching of electroactive polymers. It is well-known that, during the redox switching of these polymers, the following phenomena occurs: (i) conformational changes, (ii) income/outcome of solvent molecules with the subsequent changes in the state of swelling, (iii) injection/ejection of ions that specifically bind to the polymer, and (iv) injection/ ejection of ions to maintain the electroneutrality inside the polymer. All these facts can be taken into account by considering that both the oxidized and reduced forms of the polymer are polyelectrolytes and that the reduced and oxidized forms of the polymers are of different chemical natures. For the different forms, the different contributions to the free energy can be determined and related to the apparent formal potential. This allows the influence of each one of the aforementioned contributions to the apparent formal potential to be calculated. The model is applied to the redox switching of a well-known electroactive polymer such as poly(aniline) (Pani). From the knowledge of the changes in volume of Pani, the change in the number of water molecules, ion binding, and ion content inside the polymer can be computed.