The electropolymerization of o-aminophenol is markedly different from that of aniline and other aniline derivatives owing to the ease of generation of radical cations from the oxidation of the monomer. The growth of poly-o-aminophenol (POAP) therefore proceeds mainly through the reactions between the growing polymer and oxidized monomer molecules. The polymer matrix of POAP consists of both oxidized (quinonoid) and reduced (N-phenyl-p-phenylenediamine) repeating monomer units. The polymer is dark brown when it is oxidized at potentials greater than 0.13 V (vs. SCE), and light yellow when it is reduced at potentials lower than -0.042 V. The polymer under potentiodynamic polarization can also give rise to a completely colourless intermediate oxidation state that is made up of an equal number of oxidized and reduced units. Except for good colour retention in the oxidized state, POAP is unsatisfactory in electrochromic applications owing to rate limitation in the heterogeneous charge-transfer step. In particular. The reduction of the polymer is much slower than the reverse process of polymer oxidation.