Decomposition of urea nitrate in an initially evacuated system gave sigmoidal pressurevs. time curves. The experimental kinetic data fit the growing nuclei model with a measured enthalpy of activation of 142±12.5 kJ/mole as compared to 115±11.3 kJ/mole obtained thermogravimetrically. This higher value ofΔH‡ is explained on the basis of two factors: 1) the inhibitory effect of the product gases and 2) self heating, whose extent increasedΔH‡ by about 12.5 kJ/mole.