Abstract: Anew bromoamine-T (BAT) ion electrode made with a long-chain quaternary phosphonium salt hexadecyl triphenyl phosphonium (HTPP) as site material is described. The electrode shows remarkable improvement over the similar chloroamine-T (CAT) electrode reported in literature. Microamount of iodide in the range 0.4-64 ppb can be determined by using I- catalized BAT-H₂O₂ reaction monitored with BATion electrode. Most common ions do not interfere with the determination. The method is acceptable in the determination of protein-bound iodine. The possible reaction mechanism was proposed.