Diffusion nuclear magnetic resonance (NMR) spectroscopy was used to find the interaggregate interactions and sizes of tributyl phosphate (TBP) aggregates containing varying concentrations of uranium or zirconium and HNO3 in an n-dodecane diluent. The average diffusion coefficients of TBP species were measured using a pulsed-field gradient stimulated echo experiment with a longitudinal eddy-current delay (STE-LED). Interaggregate interactions were determined by measuring the diffusion coefficient of TBP in a sample after a series of dilutions with n-dodecane. The interaction-independent infinite dilution diffusion coefficient was also calculated from these measurements. The sizes of TBP aggregates were calculated from the infinite dilution diffusion coefficient using the WilkeChang equation. Interactions between TBP aggregates were observed to correspond to a hard sphere potential with a repulsive component. Aggregate sizes found by NMR were comparable to literature values found using small-angle neutron scattering. The diffusion of TBP in heavy organic third phases indicates that the third phase is not a bicontinuous structure like that found in traditional surfactant systems.