In addition to the optical characterization of the different forms displayed by polyaniline (PANI) thin films during their polarization in media of different acidities and in a wide range of potentials, the use of the optical multichannel analysis technique (OMA) allowed us to reach the vry fast kinetics of equilibration in a new form following the change of polarization potential. The formation of either excitons or polarons as a function of pH and the progressive passage from single to bipolaron or large polaron were stidied in particular. The results were compared with those of Raman spectroscopy; it was shown that Raman spectroscopy is particularly suitable to describe the oxdiation mechanisms (passage from benzenoid to quinoid rings), but that the protonation phenomena (passage from base to salt forms) are better analysed by OMA. We were also interested in the electronic properties of PANI. This polymer is generally used in very acidic electrolytic media; at pH = 0 or 1, the colour change mechanisms are essentially related to the chain protonation-deprotonation. The use of a less acidic medium (pH = 3) than those generally employed is a good choice for electronic devices and the properties of films polarized in this medium were compared with the results in very acidic media. Polymer degradation mechanisms during cycling were also studied and explained by the gradual formation of head-to-head couplings.