Abstract The ac impedance method has been used to investigate the conductivity of pressed HUP pellets. The activation energy for the ionic conduction process at high frequencies (30 kJ mol −1) was attributed to a simple proton hopping step. At lower frequencies (32 Hz) a Warburg diffusional impedance was observed with a much higher activation energy. This was attributed to the higher energy process of vacancy rotations needed for ion transfer over significant distances. Using a concentration cell, with HUP as the electrolyte, the thermodynamics of the hydrogen-tungsten oxide system was investigated.