Poly-[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium]chloride films on glassy carbon electrodes greatly increased the voltammetric peak currents of alizarin red S (ARS), which is an anthraquinone derivative. We propose that the adsorption sites of ARS in the polymer channel be divided into two different classes: the hydrophobic interfacial zone and the hydrated zone. The theoretical curve based on our proposed model well fits the experimental data points for the relationship between the ARS concentrations in the polymer film and in the immersing solution. The electrocatalytic effect produced by the polymer film is attributed to an effective extraction of ARS from very dilute solution (the partition coefficients on hydrophobic interfacial zones, K 1 is 2.5×107, and that on hydrated zones, K 2 is 6.3×104 in 0.2 M H2SO4). A K 1/K 2 value of 400 means that the ion-exchange ability of ARS in the hydrophobic interfacial zone is much larger than that in the hydrated zone. Although anthraquinone-2,6-disulfonate and acid violet were strongly bound to the PMPP film, ARS can not been fixed to the polymer because of a weak binding force. Using our proposed model, the ion-exchange ability of PMPP film is controlled by both of electrostatic and hydrophobic interactions, but the adsorption ability changes with steric hindrance between the anionic species and polycationic films.