The determination of iodine-129 in environmental samples is very difficult by ICP-MS due to its relatively low sensitivity, high background caused by ¹²⁹Xe impurities in the argon plasma gas and possible polyatomic interference from ¹²⁷IH₂⁺. The relatively poor abundance sensitivity of conventional ICP-MS is a problem of particular importance for ultra low-level analysis of any elements such as radionuclide elements which are adjacent in mass to elements of much higher natural abundance.In this study, in order to overcome the challenges due to the relatively poor abundance sensitivity of conventional ICP-single quadrupole MS, a newly developed triple quadrupole ICP-MS (ICP-QQQ) technique using oxygen as the reaction cell gases was applied to determine ultratrace levels of those radionuclides in aqueous samples. We improved the measurable ratio of ¹²⁹I/¹²⁷I and lower the detection limit for ¹²⁹I without loss of abundance sensitivity.