[en] The ternary thermodynamic U-Pu-O system has been studied for decades for MOX fuel applications but the phase diagram is still not precisely described mostly in the UO_2-PuO_2-Pu_2O_3 sub-system. Furthermore, uranium-plutonium mixed oxides containing high amounts of plutonium are now being considered within the scope of future nuclear reactors. Within this framework, obtaining homogeneous mixed oxides by powder metallurgy is paramount. The studied process is based on UO_2 and PuO_2 co-milling and applied to compounds with high Pu content. The objective of this study is obtaining microstructures free of local heterogeneities in the U-Pu distribution which are not suitable for research studies. Furthermore, in case of prospective irradiation application, local high Pu concentrations lead to 'hot spots' in the material influencing the fission gas release behaviour such as the thermal conductivity which may raise a number of safety issues. This study describes the effect of some fabrication parameters on the powder morphology and/or, on the final microstructure (e.g. U-Pu distribution). The co-milling, sieving and sintering steps were investigated within this scope and the resulting powders and pellets were characterised by X-ray diffraction (XRD) and optical microscopy observations, respectively. (authors)