Humic substances that preferentially adsorb at the air/water interfaces of water or aerosols consist of both fulvic and humic acid. To investigate the chemical reactivity for the heterogeneous reaction of gaseous ozone, O-3(g), with aqueous iodide, I-(aq), in the presence of standard fulvic acid, humic acid, or alcohol, cavity ring-down spectroscopy was used to detect gaseous products, iodine, I-2(g) and an iodine monoxide radical, IO(g). Fulvic acid enhanced the I-2(g) production yield, but not the IO(g) yield. Humic acid, n-hexanol, n-heptanol, and n-octanol did not affect the yields of I-2(g) or IO(g). We can infer that the carboxylic group contained in fulvic acid promotes the I-2(g) emission by supplying the requisite interfacial protons more efficiently than water on its surface.