The proton conductivity data (Fig. ) shows that the ZrHEDP samples with a high P/Zr ratio have low dependence on RH. It is the same tendency with the previous reports on sulfonated aromatic hydrocarbon polymers. The proton conductivity in this kind of random copolymer is more dependent on RH than that of Nafion. 2 However, their humidity reliance is mitigated by introducing a large amount of sulfuric acid group to a polymer, and the humidity dependence becomes comparable to that of Nafion. The properties deriving from concentrated acids seem to trigger the low dependence.