Poly(o-aminophenol) (PoAP) film was electrodeposited on an indium-tin oxide electrode. The PoAP film electrode captured dissolved cations such as Fe3+ because of the coordination ability of the film. The cation capturing ability was confirmed by the X-ray photoelectron spectroscopy and the cyclic voltammograms of the PoAP film. The redox potential of PoAP was lower than that of the Fe3+/Fe2+ redox couple, and the Fe3+ ions captured in the film were partially reduced to form Fe2+ ions. The PoAP film electrode had the Fe2+ ions, and the electrode showed a Nernstian potential response to dissolved Fe3+ ions. The response time was less than 10 s, and the response was observed in the concentration range above 10(-5) M. Although the PoAP film electrode captured Zn2+, Ni2+, and Cu2+, it showed no potential response to the corresponding cations because the corresponding reduced forms (Zn, Ni, Cu) were absent in the film. (C) 2001 The Electrochemical Society. All rights reserved.