Knudsen cell high temperature mass spectrometric measurements have been performed on neptunium dioxide. Several samples have been analysed for vaporisation in vacuum and oxidative conditions and different behaviours have been observed. The results for the vapour pressure of pure neptunium dioxide samples are discussed together with the data on americium dioxide and plutonium-americium mixed oxide samples that contained very small amounts of neptunium, in which neptunium is produced by alpha-decay from americium. In the view of extreme dilution of neptunium in the sample, Raoult's law has been applied. The ionisation efficiency curves for Np+, NpO+, and NpO2+ were recorded for constant temperature. The dissociative ionisation contribution was evaluated with respect to the composition of the gaseous phase which under Knudsen conditions is in equilibrium with the condensed phase. (C) 2010 Elsevier Ltd. All rights reserved.