Aqueous chlorine dioxide (ClO2), an alternative disinfectant for drinking water, was found to decrease gastrointestinal (GI) bioavailability of dietary iodide. It has been previously reported that subchronic exposure to ClO2 decreases thyroxine (T4) levels in nonhuman primates. In this study in vitro experiments with animal feed, isolated rat stomachs, as well as in vivo studies with intact rats, showed that ClO2 in drinking water (at in situ concentrations as low as 2 ppm) oxidizes iodide to its reactive elemental (radical) state, binding it to organic substances present in the GI tract. A single instance of acute exposure to ClO2, however, did not decrease blood iodide levels, or thyroid glandular uptake of iodine.