[en] Solvent extraction of Np with n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) from HNO₃ solution containing H₂O₂ was studied from the aspect of extraction rate. Neptunium initially in the pentavalent oxidation state can be extracted by CMPO even without the addition any redox agents, but the distribution is rather low. The H₂O₂ was found to enhance the distribution ratio. The extraction rate was almost proportional to the 1.5-power of H₂O₂ concentration. The dependence of the extraction rate on HNO₃ concentration was also examined. Reduction of Np(V) with H₂O₂ occurs rapidly in the presence of CMPO. The extraction rate is not strongly affected by the presence of tri-n-butylphosphate in the organic phase. As a diluent, decalin gives a greater extraction rate than n-dodecane. 17 refs., 9 figs