The perfluorinated polycationic polymer Tosflex @ is used for preparing polymer modified electrodes able to preconcentrate and detect the anionic complex HgCl$-, which is the prevailing inorganic Hg(II) species in sea water and other chloride media. The study of the ion-exchange voltammetric behaviour of the tetrachloromercurate(I1) complex at Tosflex-modified electrodes indicates the efficient incorporation of the analyte into the polymeric coating to which a remarkable positive shift in the reduction potential of HgCli-to Hg" is associated. The partition and selectivity coefficient values for the ion-exchange equilibrium involved are calculated from voltammetric data. When using an ion-exchange preconcentration step under open circuit conditions followed by a cyclic voltammetry detection step, a detection limit in the 10m8 M range is achieved, the exact figure depending on the scan rate employed. The application of a reducing potential during the preconcentration (i.e., using a mixed ion-exchange + Faradaic preconcentration regime) and the use of differential pulse voltammetry, lower the relevant detection limit by about three orders of magnitude. Results concerning optimization of the experimental parameters as well as lowering of the influence of possible interferents, such as copper and competing anions, are reported. Finally, an analytical procedure for exploiting the peculiar properties of Tosflex-modified electrodes for the determination of mercury in coastal waters samples is proposed and experimentally tested.