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The effect of ionic strength on the solubility of neptunium(V) hydroxide was studied under the oxidizing, C0 2 -free argon atmosphere in the pH range of 7 to 13.5 at room temperature (20-25° C). The solubility was mainly approached from oversaturation direction. The ionic strength was adjusted by NaC10 4 to 0.012, 0.05, 0.10, 0.40 or 0.80. Estimated values for the solubility product (K¡ρ) and the first and the second hydrolysis constants (ß u β 2 ) were:log K sp = -10.21 ± 0.02, log ßt = 2.91 ± 0.73, log ß 2 = 5.50 ± 0.06 at / = 0.80, log Κ sp = -9.61 ± 0.04, log jSj = 3.49 ± 0.25, log β 2 = 4.7 ±1.0 at 7=0.40, log = -8.94 ± 0.01, log β i = 2.67 ± 0.34, log β 2 = 5.74 ±0.07 at 7 = 0.10, log K sp = -8.91 ± 0.05, log ßi = 3.31 ± 0.30, log ß 2 = 5.74 ± 0.30 at 7 = 0.05 and log K sp = -8.04 ± 0.05, log ß 2 = 5.16 ± 0.94 at 7= 0.012, where 7 is the ionic strength in the molarity unit. By using these values, the specific-ion-interaction theory gives the constants at infinite dilution, 7= 0:log A? p = -8.68 ±0.26, log ßl = 3.30 ± 0.35 and log ß°2 = 5.58 ± 0.31.Neptunium hydroxide precipitates showed no X-ray diffraction peaks from 1 day to 3 months after preparation. This result indicates that the precipitates remained non-crystalline for up to 3 months.