Polyaniline (PANI), one of the most interesting members of the group of conducting polymers, has been the subject of intense experimental and theoretical studies because of its outstanding behaviour during electrochemical redox processes and during changes of pH. In this work the electrochemical doping and protonation processes of PANI are described by using in situ FTIR-ATR and a new doping system NaReO ₄ /HReO ₄ . Cyclic voltammetry shows that above ca. pH 4 only one wave response can be obtained. With increasing acidity of the electrolyte, a two-wave response develops, the splitting of the waves occurring at pH ca. 3.5–4. By in situ measurements during the base–acid transition process, a distinct picture of protonation of different forms of PANI has been obtained. A scheme for the dopant concentration, as a function of the electrode potential and pH, is presented. The results show that the amount of anions involved in the doping and dedoping processes depends on the pH and the potential. Maximum doping occurs at the intermediate oxidized metallic state. During both processes the electronic structure of the polymer and also the number of protons bound to nitrogen atoms are altered. The mechanisms of the changes under different redox and acid–base conditions are described.