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Peroxodisulfate (PDS), peroxymonosulfate (PMS), and hydrogen peroxide (H₂O₂) might coexist in a persulfate system. It leads to the mutual interference in concentration determination due to their similar structures. Simultaneous detection of the three peroxides involves limited reporting. Herein, a multi-step iodometry was established to simultaneously determine the concentrations of PDS, PMS, and H₂O₂ coexisting in a solution. Firstly, molybdate–NaHCO₃-buffered iodometry was proposed to uplift the overall detection of peroxides since the recovery rate of H₂O₂ was unexpectedly lower in the peroxide mixture than in the single H₂O₂ solution with reported NaHCO₃-buffered iodometry. Then, multi-step iodometry was proposed based on the established molybdate–NaHCO₃-buffered iodometry using the combination with catalase and revised acetate-buffered iodometry (pH 3). The multi-step iodometry determined the coexisting PMS, PDS, and H₂O₂ with the recovery rate of 95–105% and a standard deviation of ≤7% of two replicates at the individual centration of 13–500 μmol∙L⁻¹. The recovery rates of peroxides were within 95–105% at pH 3–11 and within 90–110% in the presence of Cl⁻ (0–150 mg∙L⁻¹), F⁻ (0–1.5 mg∙L⁻¹), SO₄²⁻ (0–150 mg∙L⁻¹), or NO₃⁻ (0–20 mg∙L⁻¹). The recovery rate of H₂O₂ was lowered down to 91% or 87% in the sample containing 100 mg/L Ca²⁺ or Mg²⁺, respectively, but was lifted up to 100% or 93% once pretreated by adding 0.11–1.06 g∙L⁻¹ Na₂CO₃. In the background of tap water, surface water, and ground water, peroxides were all detected in 90–110%, which indicates the applicability of multi-step iodometry in real waters.