Abstract The thermal decomposition of uranium pentabromide has been studied; and the free energy and standard heat of formation of this compound have been measured. The standard heat of formation of UBr 5 was found to be ΔH 298 f = −197·5 ± 1·3 kcal/mole. Only at temperatures below 80°C was an apparent equilibrium established, whereby a constant bromine vapour pressure could be measured: at higher temperatures, the compound decomposes irreversibly into uranium tetrabromide and liquid bromine. At 80°C the free energy of UBr 5 from UBr 4 and bromine vapour is ΔG = -0·3 kcal/mole, and by extrapolation would be zero at about 100°C. The cause of the irreversible decomposition above 80°C is not known, but it may be associated with a change in crystal structure.