Pory(o-aminophenol) (PoAP) film was electrodeposited on an indium-tin oxide electrode. The PoAP film electrode captured dissolved cations such as Fe 3+ because of the coordination ability of the film. The redox potential of PoAP was lower than that of the Fe 3+ /Fe 2+ redox couple, and Fe 3+ ions captured were partially reduced to form Fe 2+ in the film electrode. The electrode showed a Nernstian response to dissolved ferric ions. Although the PoAP film electrode captured Zn 2+ , Ni 2+ and Cu 2+ , it showed no potential response to the corresponding cations because the corresponding reduced forms (Zn, Ni, Cu) were absent in the film.