The total vapor pressure and vapor molecular weight of ammonium nitrate (NH 4 NO 3 ) 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 NH 4 NO 3 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: logP T (kPa)=(10.400±0.0002)−(4783.16±0.07)/T. The measured molecular weight of NH 4 NO 3 is 48.7g/mol for P1 cell and 50.7g/mol for P2 cell, both of which are much less than the theoretical molecular weight of NH 4 NO 3 (approximately 80.04g/mol). This significant difference in molecular weight suggests that there is disproportionation of NH 4 NO 3 sample. The mass spectroscopic results revealed that NH 4 NO 3 decomposes to NH 3 and HNO 3 ; it was interesting to note that the expected N 2 , O 2 , and H 2 O gases were not evolved during vaporization. The partial pressures of the three gas phase species (NH 4 NO 3 , NH 3 , and HNO 3 ) that were evolved during vaporization of NH 4 NO 3 sample were determined as: P1 cell: PNH4NO3/P T =0.1490, PNH3/P T =0.2911, and PHNO3/P T =0.5599, and P2 cell: PNH4NO3/P T =0.2101, PNH3/P T =0.2702, and PHNO3/P T =0.5197. The standard Gibbs energy change (ΔG°) for NH 4 NO 3 decomposition and sublimation reactions are obtained from the partial pressure results. Details of total and partial pressures of vaporization of NH 4 NO 3 and disproportionation aspects of the evolved gases are presented.