The invention is based on the preparation of an organic solution of preferably phosphonic acids and tetravalent metals salts, preferably of Zr, Ti, Sn and Ce, in organic solvents, which behaves as a solution of layered tetravalent metals salts, preferably phosphate-phosphonates, which are completely insoluble in the known solvents. This peculiarity allows an easy insertion of particles of the above compounds in the pores of porous membranes, in the matrices of those polymers, which are soluble in the same organic solvents, as well as in the membrane/electrode interfaces of fuel cells. The use of tetravalent metals salts, preferably zirconium phosphate-phosphonates, possessing high proton conductivity (in some cases higher than 10.sup.-1 S cm.sup.-1) allows the preparation of impregnated porous membranes and of nano-polymeric membranes combining good mechanical properties, and/or reduced permeability to gaseous species, with good proton conductivity. These membranes can therefore be employed in fuel cells even at temperatures higher than 80.degree. C. These membranes also possess a high catalytic activity and can therefore be employed in catalytic membrane reactors.