[en] Plutonium is the chemical element of the major concern at the nuclear legacy sites. It was previously shown that plutonium can migrate in the subsurface environment with colloidal fraction. Such colloidal migration of plutonium occurs through the formation of pseudocolloids, if plutonium is sorbed onto various natural colloidal particles, or through the formation of intrinsic colloids that and oxy-hydroxides. Additionally, it was shown that depending on the experimental conditions plutonium forms surface precipitates on various mineral colloids as a result of surface-mediated redox reactions. In this paper we aimed to study the properties related to structure and behavior of PuO_2+x·nH₂O nanoparticles with average sizes of 1.5-5 nm. As analogs to PuO₂+x·nH₂O nanosized ThO₂ and CeO₂ were synthesized under different conditions