Using ²³⁹Np-tracer, the extraction behavior of neptunium ions has been studied in the tri-n-butyl phosphate-hydrochloric acid system by the discontinuous countercurrent technique. The results indicate that 5.4N HCl is the most suitable acidity to effect a good simultaneous separation fo the three stable oxidation species in solution. Neptunium (VI), the most extractable species, was only extracted in the organic phase, Np (V) being less extractable remained in the last 10 fractions of the aqueous phase and Np (IV) was concentrated in the first fourty aqueous portions of the extractor. The separation permitted calculation of the amount of each oxidation species in the original mixture. The effect of disproportionation of Np (V), reduction of the original mixture with zinc or oxidation with KClO₃, on the extraction behavior has bean examined and the results were explained in the light of the known views.