The invention relates to the following types of composite membranes; composites or composite membranes obtained by adding a metal salt, e.g. from ZrOCl.sub.2, to a solvent, especially DMSO, for dissolving one or more polymers in an organic solvent or in aqueous systems, in addition to the subsequent precipitation in the matrix of the thus produced composite-membrane by post-treatment thereof in an acid or in a salt solution, especially phosphoric acid. The invention also relates to composites or composite membranes obtained by subsequent ion exchange of finished polymer membranes with a suitable salt cation, especially ZrO.sub.2.sup.+, wherein the polymer membrane is, optionally, swollen with an organic solvent or a mixture of organic solvent with water prior to the ion exchange and the subsequent precipitation of a low soluble salt, e.g. from Zr.sub.3(PO.sub.4).sub.4, in the membrane by post-treatment thereof in an acid or in a salt solution, especially phosphoric acid. The invention further relates to composites or composite membranes obtained by adding nano-scaled Zr.sub.3(PO.sub.4).sub.4 powder to a polymer solution, composites and composite membranes obtained according to the above-mentioned methods, wherein additional heteropoly acids are also incorporated into the polymer or membrane morphology, in addition to methods for producing said inventive polymers and membranes.