A novel water-soluble electroactive polymer, aniline pentamer crosslinked chitosan (Pentamer-c-Chi), was prepared by condensation polymerization of the terminal carboxyl groups in aniline pentamer with the amino side groups in chitosan in aqueous solution. The carboxyl groups were activated by N-hydroxysuccinimide (NHS) and N,N'-dicyclohexylcarbodiimide (I)CC). The electrochemical behavior of aniline pentamer in this kind of crosslinked polymer was studied in acidic aqueous solution by means of cyclic voltammetry (CV), UV-vis, and electron spin resonance (ESR) spectroscopy. There were three reversible redox peaks in the CV of Pentamer-c-Chi. A new emeraldine oxidization state in the form of radical cations was proposed, which was associated with the new absorption band at 370 nm in the UV-Vis spectra. The ESR of the aqueous solution of Pentamer-c-Chi showed a single Lorentzian shaped signal, which suggested the existence of radical cations. The new redox state was pH dependent and appeared only at pH < 3. The stability of radical cations could be attributed to the hydrogen bonds between radical cations, water, and chitosan. Morphological structure of the Pentamer-c-Chi can be adjusted by varying the content of aniline pentamer. (c) 2007 Wiley Periodicals, Inc.