A thin novel ionomeric plasma polymerized perfluoroallylphosphonic acid (PPPAPA) film has been developed to improve biomedical sensor ionic selectivity. PPPAPA films (average thickness 470 nm) were deposited on gold wire electrodes. The ability of PPPAPA to reject a negative organic interferant, ascorbate, was compared with the transport of a positive organic ion, dopamine, using cyclic voltammetry (CV) and chronocoulometry. From analysis of CV data, PPPAPA film-coated gold electrodes showed a 50% reduction in current density response for ascorbate and a 5% reduction in response for dopamine compared with bare gold electrodes. Apparent analyte diffusion coefficients, calculated from chronocoulometry of PPPAPA film-coated electrodes, showed a 50% reduction for ascorbate transport compared with dopamine. The results demonstrate that PPPAPA films are ion-selective and may have potential application as a biomedical sensor coating.