[en] Distribution ratios for the extraction of Zr(IV) and Hf(IV) as well as third phase formation limits for the extraction of Zr(IV) by solutions of tri-iso-amyl phosphate (TiAP) and tri-n-butyl phosphate (TBP) in n-dodecane have been measured as a function of equilibrium aqueous phase nitric acid concentration at 303 K. The concentrations of the constituents of the organic phases at different equilibrium aqueous phase Zr(IV) concentrations were determined and correlated with the third phase formation tendency of TiAP and TBP. The aggregation behaviour of TiAP and TBP in the extraction of Zr(IV) from its solution in nitric acid media (4.06-4.41 M HNO3_) has been compared using small angle neutron scattering technique. Studies indicate the higher third phase formation tendency of TBP as compared to TiAP in the extraction of Zr(IV) from nitric acid media. (author)