One amorphous and three crystalline polymorphic modlfications of UO/sub 3/ have been prepared by pyrolysis of uranyl nitrate hexahydrate. The hydration reactivity of UO/sub 3/ appears to be a function of its residual N content which is, in turn, dependent on the method, time, and temperatuee of denitration. Amorphous UO/sub 3/ is more reactive than crystalline UO/sub 3/. The initial oxidation reactivity (the O/sub 2/ sorption in 1 day at room temperature) of UO/ sub 2/ varies with the method of preparation of the UO/sub 2/. The average oxidation rate of UO/sub 2/Z after the first day decreases with increasing N content of the UO/sub 3/ from which the UC/sub 2/ was made. If soluble nitrates are removed from UO/sub 3/ by water leaching prior to reduction, the final reoxidation reactivity of the UO/sub 2/ is increased by a factor of 10. Hydration and dehydration of amomphous UO/sub 3/, without leaching, decrease the final reoxidation reactivity of the subsequent UO/sub 2/ by a factor of 10. UO/ sub 2/ made from amorphous UO/sub 3/ has a higher reoxidation reactivity than when made from crystalline UO/sub 3/ of the same N content. (auth)