Knudsen effusion mass spectrometric analysis of neptunium–plutonium mixed nitride, (Np,Pu)N, was carried out in the temperature range of 1270–2010 K. The pressures of Np(g) over the solid solutions showed a similar temperature dependence with that of Np(g) over Np(l). On the other hand, there were two stages for the temperature dependence of Pu(g) over the solid solutions. The pressures of Pu(g) at higher temperatures showed a similar temperature dependence with the case of congruent vaporization of PuN and depended on the Pu/(Np+Pu) ratio in the solid phase. The pressures of Pu(g) at lower temperatures showed a similar temperature dependence with that over Pu(l).