The thermal decomposition of solid urea hydrogen peroxide has been studied at temperatures between 333 and 358 K in flowing nitrogen atmospheres. The decomposition of freshly-prepared material follows Avrami-Erofeyev kinetics, -ln(1 - alpha) = kt(n), where n = 2 at low temperatures and low conversions, and apparently increases with degree of conversion and temperature up to 17 at 358 K. The high values of n are associated with complex melting of the adduct with liberation of oxygen. An activation energy of 1 13 kJ mol(-1) for the low-n decomposition has been calculated; the frequency factor is 10(13) s(-1). Aged material decomposes by a phase boundary process with an activation energy of 66 kJ mol(-1) and a frequency factor of 10(6) s(-1) Micrographs are presented which clarify the bulk kinetics.