Anodic to cathodic peak current ratios in cyclic voltammograms of the Fe(CN)(3-/4)couple in poly{[3-methylthiophene]-co-[1-methyl-3-(pyrrol-1-ylmethyl) pyridinium]} ionomer films were found to vary from ca 0.75 to 1.0 as the scan speed was increased. X-ray emission analysis indicated that this variation was due to competition between Fe(CN)(6)(3-/4-) and HPO42- (or H2PO4-) during the redox process. HPO42- is kinetically favoured, while Fe(CN)(6)(3-\4-) is thermodynamically favoured by the copolymer. At high scan rates, the copolymer contains a constant quantity of Fe(CN)(6)(3-/4-) because the excess of positive charge in the film produced by the oxidation of Fe(CN)(6)(4-) is balanced by HPO42- insertion. The cathodic and anodic peaks are therefore of equal height. At low scan rates, the excess positive charge is balanced by Fe(CN)(6)(3-) insertion and so the cathodic peak is 33% larger than the anodic peak. The copolymer shows excellent permselectivity to anions.