Poly(o-methoxyaniline) films doped with functionalized acids show high solubility in conventional organic solvents and a high optical contrast between the reduced and the oxidized forms. They are also more stable towards thermal treatment than when they are doped with HCl. These characteristics make them suitable for use in electrochromic displays. The electrochromic behavior of this soluble derivative of polyaniline was investigated in detail by comparing the curves obtained by the differentiation of absorbance at fixed wavelengths with simultaneously obtained voltammograms. This analysis allows the chromophore species producing color changes to be related to the redox processes taking place during the electrochemical cycling. Molar absorptivities were calculated and the effect of pH on the absorption spectra of the emeraldine form of the polymer is also reported.