Gaseous I-2 formation from the heterogeneous reaction of gaseous ozone with aqueous iodide in the presence of aqueous ferrous ion (Fe2+) was investigated by electron impact ionization mass spectrometry. Emission of gaseous I-2 increased as a function of the aqueous FeCl2 concentration, and the maximum I-2 formation with Fe2+ was about 10 times more than without Fe2+. This enhancement can be explained by the OH- scavenging by Fe3+ formed from Fe2+ ozonation to produce colloidal Fe(OH)(3). This mechanism was confirmed by measurements of aqueous phase products using a UV-vis spectrometer and an electrospray ionization mass spectrometer. We infer that such a pH-buffering effect may play the key role in general halogen activations.