UO2 fuel pellets are prepared conventionally by high- and low-temperature sintering. Both ex-ammonium uranyl carbonate (AUG) and ex-ammonium diuranate (ADU) UO2 are used in these techniques. Larger quantities of chemicals (NH, and CO2) are required for the AUG process which is a significant complication compared with the ADU process. In this work, it is aimed to search the important parameters such as theoretical density (TD) and grain size of ex-ADU UO2 pellets which are in an oxidative atmosphere at low temperature. A density of approximate to 95% TD is obtained at 1150 degrees C. The grain structure of the pellets sintered in an oxidative atmosphere is monomodal unlike the duplex grain structure of ex-AUG oxide, but it is in accord with ex-ammonium uranate (AU) oxide. The grain structure, distribution and size are compared with the ex-AUG and ex-AU conversion processes. It is shown that the grain structure development depends on both the powder properties and the sintering atmosphere. (C) 2000 Published by Elsevier Science B.V. All rights reserved.