Electrorheological fluids

An electrorheological fluid composition comprising a dispersion of disperse-phase particles formed of a sulfonated polymer possessing aromatic rings substituted with sulfonic acid groups in an electrically non-conducting oil which electrorheological fluid is characterized by the fact that the number of sulfonic acid groups in said sulfonated polymer forming said disperse-phase particles exceeds the number of aromatic rings present in said sulfonated polymer, the ratio of said disperse-phase particles to said electrically non-conducting oil is 50-500 to 100 in parts by weight, said disperse-phase particles having an average particle diameter in the range of from 0.1 to 100 .mu.m, the ratio between the number of sulfonic acid groups to those of the aromatic groups being 105 to 200 per 100 aromatic rings present in the sulfonated polymer and the water content in the dispersed phase is 1.0 to 10 parts by weight, based on 100 parts by weight of the sulfonated polymer.