Sulfonated polyetheretherketone has been used as polymer matrix for hybrid membrane formation with inorganic proton conductors . Membranes incorporating up to 40 % w/w of inorganic proton conductor, including amorphous silica, zirconium phosphate sulfophenylphosphonate and zirconium phosphate, have been prepared. The membranes have been characterised using powder X-ray di ffraction and MAS NMR spectroscopy where appropriate, and conductivity measurements performed as a function of relative humidity (RH) at 100 °C and as a function of temperature at 100 % RH. In all cases, the presence of the inorganic particles leads to an increase in conduct ivity of the polymer membrane, and is without detriment to its flexibility. For example, PEEK-S-based membranes containing 10% amorphous silica, 30 % zirconium phosphate or 40 % amorphous zirconium phosphate sulfophenylphosphonate present conductivities in the range 0.03 - 0.09 Scm -1 at 100 °C/100 % RH. Polarisation characteristics for PEEK-S membranes containing 25 and 13 % w/w of zirconium phosphate under conditions of h ydrogen/ oxygen and hydrogen/air respectively are reported, up to 120 °C.