NOVELTY - The catalytically-active composition contains at least one polysiloxane with metal ions and/or metal atoms. Metal ions and/or metal atoms are bonded to form a polysiloxane complex. The polysiloxane is obtained from a silicon-organic compound. This comprises one or a mixture of functional groups, selected from amino-, amido-, carboxyl-, sulfonic acid-, thio- or cyanide groups, containing a hetero atom. Suitable silicon-organic compounds are listed.    USE - A catalytically-active composition. The composition and a membrane electrode unit described, are used in a polymer electrolyte membrane fuel cell (PEMFC) fuel cell or a high temperature PEM fuel cell operating at -20degreesC to 250degreesC. The catalyst is alternatively used for the synthesis of organic compounds and for catalytic purification of waste gases (all claimed).    ADVANTAGE - To the present, diverse electrocatalyst systems used for commercial manufacture of fuel cell electrodes mainly comprise graphite- supported, platinum-containing materials with for the most part high precious metal content, making their manufacture costly. The transition phase between catalyst layer and electrolyte material is not adequately uniform, limiting the velocity and efficiency of ion transport in the polymer electrolyte membrane. The new composition overcomes these problems and is especially suitable for use in a membrane electrode unit. Large numbers of candidate materials are listed in the disclosure. Chemical structures are provided and discussed. Various physical forms of the catalyst and its applications are discussed.    DETAILED DESCRIPTION - The relevant metal (or its ion) is platinum, palladium, nickel, ruthenium, iron, copper, zinc, rhenium, rhodium, iridium, osmium, selenium, titanium, vanadium, chromium, manganese, cobalt, zirconium, molybdenum, cadmium, gold, silver or a mixture of them. The composition contains a component (A) conferring electrical conductivity to the mixture. This is conductive carbon or graphite, metal, a conductive polymer, preferably polyaniline, polythiophene or polypyrazole. The conductive polymer is optionally sulfonated, phosphonated, carboxylated and/or doped with one of the metals cited, or is a mixture. A flexible- and/or network-modifying polymer is included. Suitable candidates are listed. A component (B) is included to confer proton-conductivity to the composition. This is a proton-conducting cross-linked polysiloxane, a perfluorinated hydrocarbon structure with sulfonated alkyl ether side groups, polybenzimidazol or a mixture of them. Further variants based on the foregoing principles are detailed. The catalyst is obtained by heating at a temperature in the range 100degreesC-650degreesC under a protective gas or hydrogen. The catalyst contains metal atoms, metal ions or metal clusters with a diameter up to 3 nm, in a highly-dispersed form. The manufacturing process comprises: dissolving the silicon-organic compound in organic solvent, dissolving a metal compound, preferably halogenated, in a miscible organic solvent and mixing, permitting the silicon organic compound and the metal compound to react, followed by removal of the solvent(s). Further operations required, include spin-coating, hot pressing or reactive coating. INDEPENDENT CLAIMS are included for the following:    (A) the corresponding method of making the catalytic composition    (B) and the method of making a membrane electrode unit.