The understanding of the acid-base properties of conducting polymers (CPs) is essential for a rational design of devices based on this type of materials. Much of the work about polyaniline protonation equilibrium is focused on the emeraldine form. However, for the complete description of the pH effect on the electrochemical behaviour, it is required the knowledge of proton binding equilibrium to both the reduced (leucoemeraldine) and the oxidized (emeraldine) forms. In this work, the acid-base titration of polyaniline films was performed by spectroelectrochemistry at constant applied potentials so to keep the polymer in its reduced form. The values of the apparent dissociation constant, pK(a,app), result to be about 1. The analysis of the proton binding degree as a function of pH allows determining the distribution of dissociation constants. This distribution is interpreted in terms of a simple thermodynamic statistical model, that allows determining the internal dissociation constant, pK(a,int) approximate to 1.3. Comparison with the pK(a) values of related monomers reinforces the results obtained in this work. (C) 2014 Elsevier B.V. All rights reserved.