NOVELTY - An electrolyte membrane comprises a solid acid having hydrogen-bonded anion groups and a superprotonic, trigonal, tetragonal or cubic disordered phase and capable of operating at at least 100 degreesC.    USE - For use in an electrochemical device, e.g., fuel cell, battery, hydrogen separation membrane, membrane reactor (claimed).    ADVANTAGE - The solid acid compound has high proton conductivity. The solid acid structure does not suffer from reduction in the presence of catalytic materials, e.g., platinum and other transition metal elements.    DETAILED DESCRIPTION - An electrolyte membrane comprises a solid acid of formula    (a) (MxM'1-x)3H3x(PO4)2;    (b) M'3H2x((P1-xAx)O4)2;    (c) MH 1+x(PO3A)1-x(PO4)x;    (d) M3Hx((PO3A)x(PO4)x)2;    (e) MH1+x(CrO 4)1-x(PO4)x or M3Hx((CrO4)x(PO4)1-x)2;    (f) MHx(P1-xAx)F6;    (g) MxM'yHzAlF6;    (h) (M1-xHx)3AlF6, MH2AlF6 or M2HAlF6;    (i) (M1-yHy)2+x(A1-xAlx)F6;    (j) (M1-yHy)2+x(A1 -xAlx)F6;    (k) (M1-yHy)1+2x(P1-xAlx)F6;    (l) (M1-xM'x)(A1- xAlx)F6;    (m) (M1-x)HxAO4;    (n) M'H2AO4;    (o) MHAO4; or    (p) MM'HAO4.    For formulae (a), (b), (f), and (j)-(o):    M = alkali or transition metal or other functional group having a +1 charge;    M' = alkaline earth or transition metal having a +2 charge;    X, Y = at most 1;    A = silicon (Si) or germanium (Ge);    M = alkaline earth metal or transition metal having a +3 charge.    For formulae (c) and (d):    M' = alkaline earth or transition metal having a +2 charge;    A = fluorine (F) or hydrogen (H);    X = at most 1.    For formula (g):    M = alkali or transition metal or other functional group having a +1 charge;    M' = alkaline earth or transition metal having a +2 charge;    X, Y, Z = less than 3;    X+2Y+Z = 3.    For formulae (h) and (i):    M = species with a +1 charge;    M' = species with a +2 charge;    X, Y = at most 1;    A = silicon (Si) or germanium (Ge).    An INDEPENDENT CLAIM is included for a method of producing a solid acid electrolyte material by conducting a synthesis reaction of a conventional alkali containing silicate or germanate in which the alkali ion or alkaline earth ion containing reactants is replaced with a reactant containing large cation(s).    DESCRIPTION OF DRAWING(S) - The figure shows a graphical representation of the properties of a CsH(PO3H) solid acid.