Published data are used to calculate equilibrium constants for the reactions HNO3.3H2O(s)⇄HNO3(g)+3H2O(g) HNO3.H2O(s)⇄HNO3(g)+H2O(g) over the temperature range 190 to 205 K. Standard enthalpies of formation and standard entropies are calculated for the tri- and mono-hydrates. These are shown to be in reasonable agreement with earlier calorimetric measurements. The formation of nitric acid trihydrate in the polar stratosphere is discussed in terms of these equilibrium constants. ¿ American Geophysical Union 1990