Fully immobilized phosphonic acid based proton conductors, where phosphonic acid groups are tethered to cyclic siloxanes via flexible alkane spacers, are synthesized. Unlike conventional hydrated ionomers containing sulfonic acid groups, which are commonly used as separator material in PEM fuel cells, the proton conductivity of these materials occurs within a dynamical hydrogen bond network formed by the protogenic groups (phosphonic acid), which are present at very high concentrations. Conductivities of up to 2 (.) 10(-3) S cm(-1) are obtained at T approximate to 130 degrees C and RH approximate to 37%. This is only slightly higher than the conductivity of similar imidazole based systems although neat phosphonic acid has a much higher proton conductivity compared to neat imidazole. The proton conductivity of phosphonic acid is more sensitive towards immobilization at cyclic siloxanes and the corresponding restrictions for hydrogen bond formation (aggregation). (c) 2006 Elsevier B.V. All rights reserved.