The objective of this research was to develop a method suitable for the determination of aqueous concentrations of radioactive iodine as I2, I(-), and IO3(-). As one of the primary risk-drivers and contaminants of concern at nuclear waste repositories, the accurate determination of (129)I in aqueous systems is of significant concern. The redox-active nature of iodine makes its mobility and fate in the environment difficult to predict, thus underscoring the importance of species-specific determination of iodine concentrations. The developed method couples solid phase extraction with liquid scintillation counting, and scintillating anion exchange with a flow-cell detection system for a sequential measurement of each iodine species. Solid phase extraction disks were impregnated with polyvinylpyrrolidone for the selective extraction and stabilization of I2 with subsequent analysis by liquid scintillation counting. Aqueous I(-) was concentrated and measured by a previously developed flow-cell system utilizing scintillating anion-exchange resin. A subsequent chemical reduction of IO3(-) to I(-) in the effluent was used to quantify IO3(-) by the same flow-cell system. Nearly quantitative results were found for standardized single-species samples of I2 (95%), I(-) (101%), and IO3(-) (91%), respectively, while consistent measurements were obtained for multispecies samples using the developed method and algorithm.