Method for the preparation of mixed matrix membranes for the separation of fluids containing a hydrophobic perfluorinated or highly fluorinated polymer, and microporous fillers containing in origin -OH groups on their outer surface, comprising the steps of: a) surface modification of the microporous filler with fluorophilic modification agents of formula X 2 SiR f R or X 3 SiR f , where the nature of the substituent X is such that it can be exchanged to form covalent bonds between the silicon of the modifying agent and the oxygen of the -OH groups which are present on the surface of the particles to be dispersed in the matrix, R f is a hydrophobic fluorinated radical, and R is-R f , a different hydrophobic fluorinated radical, or any radical containing only C and H atoms; the modification reaction is carried out by employing the stoichiometric amount of reagent needed to react with all of the -OH groups on the outer surface of the porous fillers, or a multiple of this amount which is obtained by multiplying the stoichiometric amount of reagent for a factor in the range between 1 and 20. b) preparation of a homogeneous suspension of the modified porous filler in a solution in which: the solute is i) a hydrophobic perfluorinated polymer , or ii) a hydrophobic highly fluorinated polymer chosen from the group of homo- and co-polymers of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, perfluorodioxoles, pefluoroalkenylvinylethers, pefluoroalkoxylvinylethers, or iii) a mixture of two or more hydrophobic perfluorinated polymers and/or highly fluorinated polymers chosen from the group of homo- and co-polymers of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, perfluorodioxoles, pefluoroalkenylvinylethers, pefluoroalkoxylvinylethers; and the solvent is a fluorinated solvent or a mixture of solvents containing at least one fluorinated solvent; c) shaping of the suspension in the desired conformation of the membrane; d) removal of the solvent from the shaped suspension. a) surface modification of the microporous filler with fluorophilic modification agents of formula X 2 SiR f R or X 3 SiR f , where the nature of the substituent X is such that it can be exchanged to form covalent bonds between the silicon of the modifying agent and the oxygen of the -OH groups which are present on the surface of the particles to be dispersed in the matrix, R f is a hydrophobic fluorinated radical, and R is-R f , a different hydrophobic fluorinated radical, or any radical containing only C and H atoms; the modification reaction is carried out by employing the stoichiometric amount of reagent needed to react with all of the -OH groups on the outer surface of the porous fillers, or a multiple of this amount which is obtained by multiplying the stoichiometric amount of reagent for a factor in the range between 1 and 20. b) preparation of a homogeneous suspension of the modified porous filler in a solution in which: the solute is i) a hydrophobic perfluorinated polymer , or ii) a hydrophobic highly fluorinated polymer chosen from the group of homo- and co-polymers of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, perfluorodioxoles, pefluoroalkenylvinylethers, pefluoroalkoxylvinylethers, or iii) a mixture of two or more hydrophobic perfluorinated polymers and/or highly fluorinated polymers chosen from the group of homo- and co-polymers of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, perfluorodioxoles, pefluoroalkenylvinylethers, pefluoroalkoxylvinylethers; and the solvent is a fluorinated solvent or a mixture of solvents containing at least one fluorinated solvent; c) shaping of the suspension in the desired conformation of the membrane; d) removal of the solvent from the shaped suspension. <EOS>