Preparation of mixed ultrafine particles form PFPE microemulsion

Process for preparing dispersions containing mixed oxides having particle size lower than or equal to 50 nm and having narrow size distribution, comprising the following steps: P1) preparing a microemulsion of water in oil, the oil being a perfluoropolyether, said microemulsion being obtained using a perfluoropolyether surfactant, the aqueous phase consisting of a solution of an inorganic salt of a M1 metal; P2) preparing a M2 metal salt of an anionic perfluoropolyether surfactant, such salt being soluble in the perfluoropolyether oil used in P1, the amount by moles of surfactant being such as to fully bind to the cation and so as not to be in excess; P3) adding P2 to P1 for obtaining the reaction system containing the two metals M1 and M2; M1 in aqueous phase and M2 in perfluoropolyether oil phase; P4) adding alkali solution to P3 with contemporaneous or subsequent heating, if P3 has not yet been heated, at the temperature necessary to form the desired oxide; then cooling and separating the aqueous supernatant; M1 and M2 belong to the metals capable of giving oxides; when the two metals M1 and M2 have different valence, then M1 is the metal ion with lower valence, different from Fe(II), optionally in admixture with Fe(II), M2 is the metal ion with higher valence.