[en] Soft UO₂ microspheres could be obtained without addition of pore formers by introduction of a calcination step to convert UO₃ to U₃O₈ prior to its reduction to UO₂, The changes occurring during the calcination step and the mechanism of conversion would be useful in understanding the reason for yielding soft product, The present study deals with the conversion of UO₃ microspheres to U₃O₈ microspheres by heating up to 800 degC, at known partial pressures of oxygen, A probable reaction mechanism of conversion of UO₃ to U₃O₈ has been suggested with the diffusion of molecular oxygen through the product layer as the rate controlling step. (author)