The structural changes of ammonium diuranate (ADU) microspheres, prepared by the sol-gel route via internal gelation, were investigated during thermal treatments in oxidative and reducing conditions. In particular, in-situ simultaneous thermogravimetric analyses and in-situ high-temperature scanning electron microscopy investigations were carried out on un-doped ADU microspheres, and ADU microspheres doped with 10 mol% neodymium or 10 mol% cerium. Calcination in air caused the surface of the particles to crack, but with increasing temperature up to 900 C some healing, and additionally, shrinkage occurred. The extent of the fractures and the amount of shrinkage was, however, significantly more pronounced in a particle prepared with a tetravalent Ce precursor, as compared to particles prepared with a trivalent Nd or Ce precursor. The macroscopic behaviour could be related to the release of volatile decomposition products, which appeared to be impacted significantly by the oxidation state of the dopant precursor. After calcination, the products were identified as (Ln-doped) a-U3O8 by X-ray powder diffraction. During thermal treatment in reducing conditions a transition from a (Ln-doped) a-U3O8 phase to a (Ln-doped) UO2 x phase was observed. After exposure to a hydrogen containing gas mixture, this transition occurred rapidly in un-doped particles, and in particles prepared with trivalent Nd or Ce dopant precursors at 700 C. Despite the fast reduction reaction, severe fractures appeared in the particles above temperatures of 850 C, indicating that such behaviour is mainly attributed to sintering e ects and less to the phase transition. In contrast, a more delayed reduction reaction was observed in particles prepared with a tetravalent precursor and the described e ects appeared less severe.