The effect of the mechanoactivation on UO3 and U3O8 in agate or stainless steel vessels in air or in toluene is studied. UO2(OH)2 is the main product of UO3·H2O activation in steel vessel in air. The presence of toluene leads to strong amorphization and dispersity increase and, probably,
to the formation of U2O5. The activation of U3O8 leads to its reduction to U3O7 which relative content in the reaction mixture depends on the mechanoactivation conditions.