The use of uranium mononitride as a nuclear fuel is being considered for a number of high temperature applications. In comparison with the most often applied high temperature fuels, UO2 and UC, one finds that UN has a combination of the high melting point of UO2 and the thermal conductivity and high uranium density of UC. However, interest in UN is often dampened by qualitative indications of its low thermal stability and by lack of experimental thermodynamic data. Is is the purpose of this study, therefore, to provide a quantitative measure of the thermal stability of UN and to establish some of its thermodynamic properties.