In situ ultraviolet-visible (UV-vis) and Raman spectroelectrochemical investigations of chemically synthesized homopolymers and copolymers of aniline and o-toluidine are reported and discussed. The copolymers in their metallic state are soluble in volatile solvents such as chloroform and thus could be processed easily for electrochemical investigations. The oxidation wave in cyclic voltammograms corresponding to the emeraldine-to-pernigraniline transformation of poly(o-toluidine) (POT), unlike in polyaniline (PANT), does not show multiple peaks, and this indicates a relatively faster exchange of counterions. In situ UV-vis and Raman spectroscopy has revealed that the electrode potential at which the transformation from the emeraldine state to the pernigraniline state occurs is influenced by the fraction of o-toluidine in the feed. The spectroelectrochemical behavior of copolymers containing higher feed fractions of aniline resembles that of PANT, whereas the spectroelectrochemical behavior of copolymers with a higher feed fraction of o-toluidine resembles that of POT. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 700-708, 2009