The total vapor pressure and vapor molecular weight of ammonium nitrate (NH4NO3) were determined. The vapor pressure was determined by the torsion + effusion method, and vapor composition was determined by effusion-beam mass spectrometry. Total vapor pressures of NH4NO3 were measured by using two effusion cells with different orifice diameters over the pressure range of (10(-6) to 10(-3)) kPa, between (313 and 360) K. The equilibrium vapor pressure equation was zero-extrapolated from measurements with different orifice Knudsen cells, P1 and P2 cells, and is given as: log P-T (kPa) = (10.400 +/- 0.0002) - (4783.16 +/- 0.07)/T. The measured molecular weight of NH4NO3 is 48.7 g/mol for P1 cell and 50.7 g/mol for P2 cell, both of which are much less than the theoretical molecular weight of NH4NO3 (approximately 80.04 g/mol). This significant difference in molecular weight suggests that there is dis-proportionation of NH4NO3 sample. The mass spectroscopic results revealed that NH4NO3 decomposes to NH3 and HNO3; it was interesting to note that the expected N-2, O-2, and H2O gases were not evolved during vaporization. The partial pressures of the three gas phase species (NH4NO3, NH3, and HNO3) that were evolved during vaporization of NH4NO3 sample were determined as: P1 cell: P-NH4NO3/P-T = 0.1490, P-NH3/P-T = 0.2911, and P-HNO3/P-T = 0.5599, and P2 cell: P-NH4NO3/P-T = 0.2101, P-NH3/P-T = 0.2702, and P-HNO3 P-T = 0.5197. The standard Gibbs energy change (Delta G degrees) for NH4NO3 decomposition and sublimation reactions are obtained from the partial pressure results. Details of total and partial pressures of vaporization of NH4NO3 and disproportionation aspects of the evolved gases are presented. (c) 2010 Elsevier Ltd. All rights reserved.