Isothermal decomposition of un-irradiated and pre-gamma-irradiated dehydrated uranyl acetate has been investigated at different temperatures between 573 and 593 K. Irradiation enhances the rate of decomposition without modification of the mechanism of thermal decomposition which has been shown to proceed by a nucleation and growth mechanism, both for un-irradiated and pre-gamma-irradiated samples of uranyl acetate. The enhancement of the decomposition was found to increase with an increase in the gamma-ray dose applied to the sample and may be attributed to an increase in the number of point defects and formation of additional nucleation centers generated in the host lattice.