The synergistic effect of tri- n-octylmethylammonium chloride (capriquat, QCl) on the solvent extraction of Np(V) by thenoyltrifluoroacetone (TTA, HT) was investigated over the pH range 2–8. The reaction for the extraction of Np(V) was shown to be: NpO 2 + + T − + 2(QT) o = (NpO 2T2QT) o . The logarithm of the extraction constant, log K ex, was 2.3. It was ascertained that the interaction between QCl and TTA during extraction, (QCl) o + (HT) o = (QT) o + H + + Cl −, seriously affected the extraction of Np(V) and that the effective extraction of Np(V) could be obtained in the presence of excess HT over QCl.