Abstract: A PVCmembrane bismuth (Ⅲ) ion-selective electrode based on the triheptyl-dodecyl-tetraiodobismuthate (Ⅲ) ion pair is described. In a solution of 0.15 MKI, the electrode exhibits Nernstian response for bismuth (Ⅲ) ion over the concentration range from 1 × 10^-2Mto 3×10^-6Mwith a slope of 60 mV (at 25℃). The detection limit is down to 1.3× 10^-6M. Membrane potentials are independent of pH 1-4. The potentiometric selectivity coefficient of some anions have been measured. The preparation of the electrode and optimum conditions for its use are discussed.