The synthesis and characterization of poly(thiophenylenesulfonic acid), a novel class of polyaromatic electrolyte possessing up to 2.0 sulfonic acid groups per phenylene unit (m=2.0), are described. 4-(Methylsulfinyl)diphenyl sulfide (I) was polymerized in sulfuric acid upon heating (<140 degrees C) or in the presence of SO3 to yield a sulfonated poly(sulfonium cation) (4), which can be converted to the corresponding sulfonated poly(thiophenylene). The precursor method using the soluble poly(sulfonium cation) makes it easy to control the sulfonation reaction in a homogeneous system. The resulting poly(thiophenylenesulfonic acid), unlike the nonsubstituted polymer, is soluble in water and methanol and can form a transparent film. The sulfonated polymer exhibits a good water affinity and an excellent proton conductivity due to the high carrier concentration. The highest conductivity to (sigma=4.5x10(-2) Scm(-1)) was achieved for the polymer with m=2.0 at 80 degrees C.