NOVELTY - Oligo- or polymer siloxane containing a phosphonic acid group with one or more units (I), is new.    USE - The oligo- or polymer containing phosphonic acid is useful in membrane-, film-, composite- or compound material; and for reducing swelling of aromatic polyphosphonic acid and in-situ formation of polyvalent metal-polyphosphonate polyelectrolyte-polyphosphonic acid-blend-membrane by ionic cross linking. The oligo- or polymer containing phosphonic acid is useful for bonding metal ion such as titanium, zinc, tin, magnesium, germanium, zirconium, aluminum, hafnium, earth alkaline metal, rhodium, palladium, platinum, gold, silver and/or actinide; and contact mediation or -improvement of material such as metals, plastics and further materials e.g. apatite, where the contact mediation or improvement between the material results in single material class and/or more of the mentioned material classes. The oligo- or polymer containing phosphonic acid is useful as corrosion inhibitor in metal coating and as acid catalyst. The membrane is useful in fuel cell or as a membrane in separation technique or as separator in galvanic or electrochemical cell (all claimed).    ADVANTAGE - (I) exhibits high thermal-, chemical- and mechanical stability. (I) provides flexibility for the membrane.    DETAILED DESCRIPTION - Oligo- or polymer siloxane containing a phosphonic acid group with one or more units (I) of formula ((Si(A1)xa(B1)ya(Y1))ka-(Si(A)x(B2)y(Y2)-(O)(4-(x+y)/2))ka), is new.    Y1, Y2 = -((Si(A2)xb(B2)yb)kb) or (Si(A3)x(B3)y-(O)(4-(x+y)/2))kc);    A, A1-A3 = (R1(P(O)(OH)2)n);    B, B1-B3 = (R2(P(O)(OH)2)m); either    x, y, xa-xc, ya-yc = 0-2; or    x+y, xa+ya, xb+yb, xc+yc = 3;    m, n = 0-2;    k = greater than or equal to2;    ka-kc = 0-4, preferably 0;    R1 = di- or polyvalent aromatic residue (optionally further contains one or more residue of (P(=O)(OH)2) or heteroatoms); and    R2 = aryl- or alkyl (optionally further substituted by one or more residues of (P(=O)(OH)2) or one or more heteroatoms.    Where Y1 and Y2 can be bonded to Si-atom, A3 or O-atom and/or Si-atom or group A2 to a Si-atom of (I). INDEPENDENT CLAIMS are included for:    (1) the preparation of the oligo- or polymer siloxane containing phosphonic acid group;    (2) a phosphonic acid silylester- and/or -alkylester group containing oligo- or polymer siloxane, produced by the above procedure;    (3) an oligo- or polymer siloxane containing phosphonic acid obtained by the above procedure;    (4) a mixture comprising at least an oligo or polymer containing phosphonic acid group and/or at least an phosphonic acid silylester- and/or alkyl ester comprising oligo or polymer siloxane and at least an polymer;    (5) a membrane, film, composite or compound material comprising at least phosphonic acid group that containing oligo- or polymer siloxane and/or at least an phosphonic acid silylester- and/or -alkylester comprising oligo- or polymer siloxane or mixture; and    (6) a fuel cell comprising at least an membrane or at least a oligo- or polymer siloxane containing phosphonic acid group.