We focus our attention here on semisquaric acid, which is known to show high acidity, as a new proton dissociating group for proton exchange membranes (PEMs). The introduction of a squaric acid group into aromatic polymers was concluded by the reaction of lithiated aromatic polymers and diisopropoxy squarate, followed by treatment with hydrochloric acid. A resulting polyphenylsulfone membrane with the squaric acid group introduced (PPSf-SQ, IEC = 4.1 meq.g(-1)) showed proton conductivity of 1.0 x 10(-1) S.cm(-1) at 80 degrees C under 95% relative humidity, which indicates that the semisquaric acid has the potential to become an alternative proton-conducting group for PEMs.