Sulfonated polyaniline (SPAN) films were studied using simultaneous potentiodynamic, ellipsometric, spectroelectrochemical, and quartz crystal microbalance experiments. Ellipsometric experiments show that there is a small variation of film thickness (similar to 2%) between the reduced and oxidized states. The role of ionic transport during the redox reaction is emphasized in order to obtain information about all chemical species involved in the process occurring in the conducting polymer film. Quartz crystal microbalance results show the relevant participation of cations in the charge compensation process. The spectroelectrochemical study shows spectral changes accompanying the oxidation/reduction process of SPAN as the increase/diminution of the polaron band at 400 nm and another band at higher wavelengths as a function of potential. These changes were shown to be reversible.