A spectrophotometric flow injection procedure for the determination of sulphite in potassium chloride brine is described. The reagent used was the organic disulphide 2,2′-dinitro-5-5′-dithiodibenzoic acid and the effect of the reaction pH on the kinetics is discussed. A laboratory method for the determination of sulphite in the range 0.1–20 mg l^–1 is presented for three matrices; water, standard potassium chloride solutions and potassium chloride brine. The robustness of the method is discussed in terms of sample pH, sample temperature and potassium chloride concentration. A modified procedure for on-line analysis with a linear range of 3–100 mg l^–1 is described and the results are compared with those obtained off-line by iodimetric titration. The relative standard deviation for 168 results (triplicate determinations of a 62.3 mg l^–1 sulphite standard) over a 7 d cycle was 2.1%. Results are also presented for a 21 d on-line trial.