NOVELTY - Halogen-free, oligomeric or polymeric phosphonic acid (I), is new.    USE - (I) or a polymer blend is useful in fuel cells and in film-, composite or compound material. The membrane is useful in fuel cell or as a membrane for the separation technique (all claimed). (I) is useful in polyelectrolyte membrane.    ADVANTAGE - (I) is halogen-free (claimed) and provides highly pure polyphosphonic acid membrane.    DETAILED DESCRIPTION - Halogen-free, oligomeric or polymeric phosphonic acid (I) of formula ((O)m-(R1(X)n(Y1)o)pR1(X)q)r-((O)s-R5(X)t)v, is new.    X = -P=O(OH)2;    Y1 = carbonyl (preferred) or sulfonyl;    R1, R5 = di- or polyvalent (optionally heteroatom-free or heteroatom-comprising aromatic radicals);    m, o, s = 0 (preferred) or 1;    n = 0 or an integer 1-8, preferably 1;    q, t = 0 or an integer 1-8; either    r, v = 0-1; or    r+v = 0.95-1.05; and    p = 0 or greater than 1.    Where the radicals X are substituents of R1 and R5, and the radicals Y1 are parts of a polymer or oligomer chain; and where n and s are not 0.    INDEPENDENT CLAIMS are included for:    (1) the preparation of (I);    (2) a halogen-free, oligo- or polymer, silylated phosphonic acid (IV) of formula (((O)m-(R1(Z)n(Y1)o)pR1(Z)q)r((O)s-R5(Z)t)v);    (3) the preparation of (IV) comprising reacting oligo- or polymer compound (II) of formula (((O)m-(R1(Y1)o)pR1)r((O)s-R5)v) with a halogenating agent at -20degreesC to 140degreesC in an inert solvent to obtain an haloaromatic oligo- or polymer compound (III) of formula (-((O)m-(R1(Hal)n(Y1)o)pR1(Hal)q)r((O)s-R5(Hal)t)v-); and reacting (III) in a nitrogen free solvent or its mixture with O-silylated oxygen compound of the trivalent phosphors at above 100degreesC in presence of a catalyst comprising at least a metal such as platinum of periodic system of the element to obtain (IV);    (4) a halogen-free mixture comprising (I) (0.1-99.9 wt.%) and an nitrogen-free solvent with a boiling point of greater than 150degreesC;    (5) a film-, composite or compound material comprising (I);    (6) a polymer blend comprising at least (I) and at least an polymer;    (7) a membrane comprising (I) or the polymer blend; and    (8) a fuel cell comprising at least (I) or polymer blend.    Z = -P(=O)(OSiR2R3R4)(OSiR6R7R8); and    R2-R4, R6-R8 = alkyl, alkenyl, aryl aliphatic, cycloaliphatic or hetero group containing alkyl.