The formation of iodo-trihalomethanes (I-THMs) such as iodoform (CHI3) during oxidative treatment of iodide-containing drinking waters can be responsible for taste and odor problems. I-THMs are formed by reactions of hypoiodous acid (HOI) with natural organic matter. HOI is quickly formed from naturally occurring iodide (I-) by oxidation with ozone, chlorine, or chloramine. The kinetics of reactions of HOI with organic model compounds as well as the resulting CHI3 formation were measured. Substituted phenols, phenol, and, to a smaller extent, alpha-methyl carbonyl compounds were found to be reactive toward HOI and also to yield CHI3. Resorcinol (m-hydroxyphenol) had the highest yield of CHI3. The kinetics of I-THM formation were also measured in natural waters which were oxidatively treated with ozone, chlorine, or chloramine. When ozone was used, no I-THMs were detected and greater than or equal to 90% of I- was transformed to IO3-. Chlorine led to the formation of both IO3- and I-THMs. With increasing chlorine doses, the CHI3 formation decreased, whereas IO3- formation, as well as the formation of classical THMs such as chloroform, increased. In chloramination processes, I-THMs (especially CHI3) were the main products. The CHI3 formation in the oxidation of natural waters increased in the order O-3 < Cl-2 < NH2Cl.