In order to investigate the selectivity of Pt/PANI/HRP biosensor, Mg2+, Zn2+, Ca2+, K+, Fe2+, and Na+ in tap water (drinking water) were studied by mixed and separate solution methods. For the mixed solution method, the concentration of the interfering cation was 1.0 ppm while that of the analyte was 0.5 ppm. In the case of separate solution method, both the analyte and interferent concentrations were equal (1.0 ppm). The amperometric selectivity coefficients (K) and response ratio (R) were calculated according to Marcca and Wang (1995). The K and Rij values in Table 3 showed that Fe3+ is a strong interferent (with Cd2+, Pb2+, and Cu2+) while the interference of the other cation were relatively insignificant.