Use of butyraldehyde for selective reduction of Neptunium has been reported earlier. As there is no model available to Account for its extraction behaviour in the conventional purex system, the reported experimental results could be explained only on the qualitative basis. The reported data were analyzed to determine its solvation number and an empirical model was, developed for estimation of its distribution coefficients under purex conditions. The reported-counter-current extraction run in mixer-settlers-was also computer simulated using an in-house developed purex code SIMPSEX. The simulated profiles were compared with the experimental stage profiles and a satisfactory agreement was observed.