Abstract: In a dil.H2SO4 medium,the oxidation of I- ion by KBrO3 was catalyzed by trace amount of vanadium(Ⅴ),and the potentiol change of I2/I-3 was detected by iodide ion selective electrode.It was found that the difference of potentials between the non-catalyzed reaction and catalyzed reaction (ΔE=E0-Et) was kept in linear relationship with the concentration of vanadium(Ⅴ) in the range from 0.015-0.10 mg of V(Ⅴ) per L,with a detection limit of 3.2×10-7g·L-1.It was also shown that the catalytic reaction was a 1st-order reaction,and its rate of reaction was expressed by ΔE.In its application to the determination of vanadium in steel samples,a preliminary anion ion-exchange separation of vanadium as VO3-4 on 717 strong-basic anion exchange resin from a slightly basic solution of pH 7.0-8.2.The vanadium (VO3-4) was eluted from the resin with 1.0 mol·L-1 H2SO4,and determined by the proposed method.The results obtained were checked quite well with the standard values,and values of RSD′s (n=9) were less than 5%.