Abstract: During the preparation of the iodide-iodate solution I^- was oxidized,which caused large deviation of calculated segregation index from experimental value.In this paper, the concentrations of I^-₃,I^- and I₂ in as-prepared solution were determined through concentration analysis of I^-₃ by spectrophotometer, and then the oxidized amount of I^- was calculated.A modified expression of segregation index for iodide-iodate reaction system was deduced and verified by experiments, indicating that the deviation between experimental results and the modified expression was within ±15%.