The anion-exchange behavior of neptunium ions was studied by the use of ²³⁹Np. Neptunium adsorbed on the column of Dowex-1 resin was eluted with nitric acid. The shape of the resulting curves was studied by changing the experimental conditions of eluting solution and by using exchanger with different degree of cross linkage. The oxidation states of neptunium were assigned by coprecipitation tests with sample drops taken from the vicinity of elution peaks. From the position of peaks of the elution curves, K_d values for Np(V) and Np(VI) were calculated. The K_d value of Np(V) is a second power dependence on the acidity of eluting solution, whereas that for Np(VI) gives first power dependence. Such relationships for Np(V) and Np(VI) coincide with those of Pa(V) and U(VI), respectively. K_d values of actinide ions in the oxidation state of IV, increase with increase of atomic number but those of (V) and (VI) give reverse relation; e. g. the K_d values for Np (V) and Np(VI) are smaller than those of Pa(V) and U(VI), respectively.
Np(VI) is eluted out before Np(V) with nitric acid of the concentration higher than 6M, but the reverse relationships hold for lower concentrations. K_d values give linear relationship with the degree of cross linkage of the anion exchanger up to 8 when 6M nitric acid was used as eluant.