A rapid and precise method for the determination of sulfite ion by isotachophoresis was developed. The leading electrolyte used was 0.01 M L-histidine hydrochloride in 30% aqueous acetone (pH 4.0) and the terminating electrolyte was 0.01 M hexanoic acid (pH 3.5). This method also successfully detected sulfate, the oxidation product of sulfite, which may be formed in various pharmaceutical procedures including sterilization. The limits of detection were 0.2 mM as sulfite and 0.1 mM as sulfate. No pretreatment of the sample was required. This procedure could be applied to the simultaneous determination of sulfite and sulfate and should be useful in pharmaceutical analysis.