We present a comparative vibrational study of leucoemeraldine, emeraldine, and pernigraniline bases: fully reduced, half oxidized, and fully oxidized forms of polyaniline, respectively. By performing a general vibrational calculation based on the symmetrized dynamical matrix, we determine the force constants, the potential-energy distribution, and the Cartesian displacements for the three forms of polyaniline and associated model compounds. We discuss the assignment of the fundamental Raman and ir vibrational modes of the polymers. The modifications of the frequencies and consequently of the main force constants observed from one compound to the other are analyzed by considering the quinoid and aromatic characters along the chain. In this way, we determine a force field with physical significance which may be used to interpret the electronic modification between neutral and protonated as well as photoexcited emeraldine forms. This comparative analysis demonstrates the important changes of the electronic distribution around the nitrogen atom, which plays a major role in the conduction mechanism in this class of conducting polymers.