Lead oxide (PbO₂) can be an important form of lead mineral scale occurring in some water distribution systems. It is believed to be formed by the oxidation of lead-containing plumbing materials by free chlorine. Its reactivity in water, however, has not been well studied. Iodide is also found in source drinking waters, albeit at low concentrations. Consideration of thermodynamics suggests that iodide can be oxidized by PbO₂. In this investigation, iodide ion was used as a probe compound to study the reduction of PbO₂ and the formation of iodoform, which has been predicted to be a carcinogen, in the presence of natural organic matter (NOM). The reduction of PbO₂ by iodide can be expressed as PbO₂ + 3I⁻ + 4H⁺ → Pb²⁺ + I₃⁻ + 2H₂O, and the reaction kinetics has been determined in this study. In the presence of NOM, I₃⁻ reacts with NOM to form iodoform and its concentration is proportional to the NOM concentration. Our results indicate that PbO₂ is a very powerful oxidant and can possibly serve as an oxidant reservoir for the formation of iodinated disinfection byproduct through a novel reaction pathway.