Production of actinide oxide powder via dry thermal decomposition of corresponding oxalates is currently used at industrial scale at temperatures exceeding 500 °C. Altough it is simple, this method presents some disadvantages such as high decomposition temperature with direct effect on the surface area, pre-organised morphology of the nanoparticles affecting the sintering behaviour, etc. We have recently proposed the hydrothermal decomposition of AnIV-oxalates as a simple way to produce reactive actinide oxide nanocrystals. The method could be easily applied at low temperature (95-250 °C) in order to generate highly crystalline nano-AnO2. We present here the formation conditions of AnO2 (An= Th, U, Np, and Pu) and some associated solid solutions, their stability, and grains growth during thermal treatment. The involvement of the water molecules in the oxalate hydrothermal decomposition mechanism has been demonstrated by the isotopic exchange reaction during the thermal decomposition of the hydrated oxalate in H217O through MAS NMR and Raman techniques.