Abstract: The solubility of Np(Ⅳ) in simulated underground water and redistilled water has been measured with the variations of pH(6～12) and storage time(0～100 d) in the presence of reductants(Na 2S 2O 4, metallic Fe). All experiments are performed in a low oxygen concentration glove box containing high purity Ar(99.99 %), with an oxygen content of less than 5×10 -6 mol/mol . Experimental results show that the variation of pH in solution has little effect on the solubility of Np(Ⅳ) in the two kinds of water; the measured solubility of Np(Ⅳ) is affected by the composition of solution; with Na 2S 2O 4 as a reductant, the solubility of Np(Ⅳ) in simulated underground water is (9 23±0 48)×10 -10 mol/L, and that in redistilled water is (8 31±0 35)×10 -10 mol/L; with metallic Fe as a reductant, the solubility of Np(Ⅳ) in simulated underground water is (1 85±0 56)×10 -9 mol/L, and that in redistilled water is (1 48±0 66)×10 -9 mol/L.