An inexpensive and durable polyelectrolyte composition includes both an aromatic polymer containing carbonyl linkages and/or sulfonyl linkages in the backbone chain and bearing cation-exchange groups and a fused salt exhibits a high ionic conductivity even in the absence of water or a solvent. The aromatic polymer is preferably an aromatic polyether sulfone comprising specific structural units and bearing cation-exchange groups, an aromatic polyether ketone comprising specific structural units and bearing cation-exchange groups, an aromatic polyether sulfone block copolymer consisting of at least one hydrophilic segment bearing cation-exchange groups and at least one hydrophobic segment free from cation-exchange groups, and/or an aromatic polyether ketone block copolymer consisting of at least one hydrophilic segment bearing cation-exchange groups and at least one hydrophobic segment free from cation-exchange groups. The use of such a block copolymer as the aromatic polymer gives polyelectrolyte compositions which are excellent in maintenance of structure even at high temperature.