We report an investigation of the reaction of isotope-substituted hydrated iodide I(HO)- with ozone 16O3 to examine the involvement of the water molecules in the oxidation reactions that terminate with the formation of IO3-. Experimentally, we studied the reaction in the gas phase as elementary reactions using a radio-frequency (RF) ion-trap combined with a quadrupole mass spectrometer (QMS). In approximately 1.2% of the reactions of I(HO)- and 16O3, the 18O atom is found to appear in iodine oxide anions, thus giving evidence for a close involvement of the water molecule in a non-negligible number of the reactions towards IO3-. As a part of the experimental investigation, the reaction rate constant for the exchange reaction I(HO)- + HO → I(HO)- + HO at 300 K was found to be (1.3 ± 0.1) × 10-8 cm3 s-1. Quantum chemical calculations are exploited to establish the energetic difference between I(HO)- and I(HO)-.