Iodoorganic compounds are known for taste and odor problems in drinking waters. Iodoorganic compounds can be more toxic than chloro- and bromoorganic analogues. There is therefore a need to control the formation of iodoorganic compounds in water treatment plants during ozone oxidation. The ozonation of iodide (I₋) at environmental concentrations in water, below 100 μg/L, is known to yield only iodate (IO₃⁻). Nonetheless, there is few knowledge at higher iodide concentrations such as mg per L levels found in coastal areas and in surface waters contaminated by the hydraulic fracturing. Therefore, we have studied here the reaction of a high concentration of I⁻, of 20 μM, 2.54 mg/L, during ozonation and O₃/H₂O₂ oxidation under various conditions. Results show the production of 20 % hypoiodous acid (HOI) and 80 % IO₃⁻ in deionized water, whereas in natural waters results show the production of 8.0–14 % HOI, 75–89 % IO₃⁻ and 2.7–10 % unknown iodine compounds. The addition of humic acids increased the production of unknown iodine compounds, possibly formed by reaction of natural organic matter and HOI. Overall, our findings show that ozonation of I⁻ at mg/L levels produces HOI, raising the possibility of forming iodinated by-products, whereas the O₃/H₂O₂ process can suppress the production of HOI.