Abstract The solvent extraction behavior of zirconium in the HN03-tributyl phosphate (TBP) system can be explained based on the existence of four principal aqueous species, Zr4+, ZrOH3+, Zr3(OH)8+ 4, and oxo-polymers. The Zr4+ and ZrOH3+ species are extractable and are in equilibrium with inextractable Zr3(OH)8+ 4. The oxo-polymers are formed by heat, are-inextractable, and are not: in equilibrium with the other species. The aqueous equilibria and their equilibrium quotients have been previously determined. In the present study, these equilibria were used along with both tracer and macro zirconium concentrations (oxo-polymers excluded by extraction and back scrubbing) to determine the distribution equilibrium constants for both the Zr4+ and ZrOH3+ ions. The four equilibrium constants give excellent fits to both tracer and macro-zirconium distribution data. The concentrations of the extractable zirconium species which are calculated from the equilibria have been used to begin examining the extraction kinet...