Thermogravimetric and thermodifferential analyses were used to characterize the behavior during heating of ammonium diuranate (ADU) powders and microspheres. On the basis of a proposed precipitating mechanism of ADU, the main conversion stages of ADU into U3O8 are taken into consideration determining the temperature intervals in which they take place. The approximate composition of intermediate products was also determined. The presence of impurities in the raw product affects the temperature at which different decomposition processes take place.