From the measured molecular weight of vapor over NH(4)NO(3)(c) by the simultaneous torsion-effusion/mass loss method, the partial pressure of NH(4)NO(3)(g) was evaluated over the range 321-360 K; in addition, the presence of the gaseous nitrate was verified by effusion-beam mass spectrometry. Third-law calculations of the sublimation enthalpy of NH(4)NO(3) were made with estimated molecular constants, yielding DeltaH(298)(o)(sub) = 20.9 +/- 2 kcal mol(-1), as compared to a less-reliable second-law value of 23.2 +/- 3 kcal mol(-1). We prefer the third-law value, which leads to the enthalpy of formation Delta(f)H(298)(o) (NH(4)NO(3)(g)) = -66.5 +/- 2.1 kcal mol(-1) when combined with the known Delta(f)H(298)(o) value of NH(4)NO(3)(c).