A significant fraction of the total organic halogen (TOX) in drinking water cannot be accounted for by known specific disinfection byproducts (DBPs). Extensive studies have been conducted to investigate factors affecting the formation of trihalomethanes and haloacetic acids, the two groups of regulated DBPs. However, the formation and control of the other, unidentified halogenated DBPs, which may have potential health risks, have not been well studied. This research evaluated the effect of reaction time, pH, dosage, and temperature on the formation of unknown TOX (UTOX) during chlorination and chloramination. Results showed that the formation of unknown DBPs significantly decreased as chlorination or chloramination pH increased. Greater conversion of UTOX to measurable byproducts occurred with increasing reaction time, pH, dosage, and temperature during chlorination. The UTOX-to-TOX ratio increased with reaction time but decreased with pH and dosage during chloramination. Study results can help water suppliers better control the formation of unknown DBPs as well as regulated DBPs, thus reducing health risks associated with halogenated DBPs in drinking water.