Metal(IV) phosphonates with layered structures of both alpha-type (M-IV(O3P-R)(2).nH(2)O) and gamma-type ((MPO4)-P-IV(O2P(OH)R). nH(2)O) can be regarded as inorgano-organic polymers in which organic moieties (R) are bridged through phosphorus atoms to an inorganic two dimensional matrix. If R contains a protogenic function (e.g. -COOH, -PO3H, -SO3H, -NH3+), these compounds are proton conductors. By a suitable choice of R, the proton conductivity can be increased almost continuously to values as high as 0.05 S cm(-1) which are comparable with those of NAFION membranes. Since the thennal stability is also very good (180-200 degrees C) and thin layers can be obtained from colloidal dispersions, these compounds are very promising for use in solid state electrochemical devices at medium temperature.