Abstract: In a dil.H2SO4 medium,the oxidative decoloration of methyl orange by KBrO3 was found to be inhibited remarkably by trace amounts of Fe(Ⅲ) ion.Moreover,sensitive adsorptive polarographic waves were observed to be produced by both the reductive form and oxidative form of methyl orange at -0.59 V and -0.50 V (vs.SCE) respectively at the dropping mercury working electrode in ammoniacal medium.By measuring the peak current (Ip) of the sample solution (i.e.,in presence of ferric ion) and of the blank solution (i.e.,in absence of ferric ion) at -0.59 V (vs.SCE),content of ferric ion in the sample was determined indirectly according to the value of ΔIp.It was found that under the optimum condition,linear relationship between ΔI′p and concentration of Fe(Ⅲ) was obtained in the range 0.008 0-0.088 mg·L-1,and the detection limit was found to be 2.76×10-6g·L-1.